Implements `/clusters` API for REST Proxy

# What is currently missing?

The `/clusters` APIs are not implemented for the REST Proxy

# How could this be improved?

Implement these APIs:

Verb | URL
--- | ---
GET | [`/clusters`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters)
GET | [`/clusters/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id)
GET | [`/clusters//acls`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-acls)
POST | [`/clusters//acls`](https://docs.confluent.io/platform/current/kafka-rest/api.html#post--clusters-cluster_id-acls)
DELETE | [`/clusters//acls`](https://docs.confluent.io/platform/current/kafka-rest/api.html#delete--clusters-cluster_id-acls)
GET | [`/clusters//broker-configs`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-broker-configs)
POST | [`/clusters//broker-configs:alter`](https://docs.confluent.io/platform/current/kafka-rest/api.html#post--clusters-cluster_id-broker-configs-alter)
GET | [`/clusters//broker-configs/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-broker-configs-name)
PUT | [`/clusters//broker-configs/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#put--clusters-cluster_id-broker-configs-name)
DELETE | [`/clusters//broker-configs/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#delete--clusters-cluster_id-broker-configs-name)
GET | [`/clusters//broker//configs`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-brokers-broker_id-configs)
POST | [`/clusters//broker//configs:alter`](https://docs.confluent.io/platform/current/kafka-rest/api.html#post--clusters-cluster_id-brokers-broker_id-configs-alter)
GET | [`/clusters//broker//configs/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-brokers-broker_id-configs-name)
PUT | [`/clusters//broker//configs/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#put--clusters-cluster_id-brokers-broker_id-configs-name)
DELETE | [`/clusters//broker//configs/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#delete--clusters-cluster_id-brokers-broker_id-configs-name)
GET | [`/clusters//topics//configs`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics-topic_name-configs)
POST | [`/clusters//topics//configs:alter`](https://docs.confluent.io/platform/current/kafka-rest/api.html#post--clusters-cluster_id-topics-topic_name-configs-alter)
GET | [`/clusters//topics//configs/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics-topic_name-configs-name)
PUT | [`/clusters//topics//configs/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#put--clusters-cluster_id-topics-topic_name-configs-name)
DELETE | [`/clusters//topics//configs/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#delete--clusters-cluster_id-topics-topic_name-configs-name)
GET | [`/clusters//brokers`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-brokers)
GET | [`/clusters//brokers/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-brokers-broker_id)
DELETE | [`/clusters//brokers/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#delete--clusters-cluster_id-brokers-broker_id)
GET | [`/clusters//brokers//partition-replicas`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-brokers-broker_id-partition-replicas)
GET | [`/clusters//consumer-groups`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-consumer-groups)
GET | [`/clusters//consumer-groups/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-consumer-groups-consumer_group_id)
GET | [`/clusters//consumer-groups//consumers`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-consumer-groups-consumer_group_id-consumers)
GET | [`/clusters//consumer-groups//consumers/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-consumer-groups-consumer_group_id-consumers-consumer_id)
GET | [`/clusters//consumer-groups//consumers//assignments/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-consumer-groups-consumer_group_id-consumers-consumer_id-assignments)
GET | [`/clusters//consumer-groups//consumers//assignments//partitions/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-consumer-groups-consumer_group_id-consumers-consumer_id-assignments-topic_name-partitions-partition_id)
GET | [`/clusters//topics`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics)
POST | [`/clusters//topics`](https://docs.confluent.io/platform/current/kafka-rest/api.html#post--clusters-cluster_id-topics)
GET | [`/clusters//topics/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics-topic_name)
DELETE | [`/clusters//topics/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#delete--clusters-cluster_id-topics-topic_name)
GET | [`/clusters//topics//partitions`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics-topic_name-partitions)
GET | [`/clusters//topics//partitions/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics-topic_name-partitions-partition_id)
GET | [`/clusters//topics/-/partitions/-/reassignment`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics---partitions---reassignment)
GET | [`/clusters//topics//partitions/-/reassignment`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics-topic_name-partitions---reassignment)
GET | [`/clusters//topics//partitions//reassignment`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics-topic_name-partitions-partition_id-reassignment)
GET | [`/clusters//topics//partitions//replicas`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics-topic_name-partitions-partition_id-replicas)
GET | [`/clusters//topics//partitions//replicas/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-topics-topic_name-partitions-partition_id-replicas-broker_id)
GET | [`/clusters//brokers/-/tasks`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-brokers---tasks)
GET | [`/clusters//brokers//tasks`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-brokers-broker_id-tasks)
GET | [`/clusters//brokers/-/tasks/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-brokers---tasks-task_type)
GET | [`/clusters//brokers//tasks/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-brokers-broker_id-tasks-task_type)
GET | [`/clusters//remove_broker-tasks`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-remove-broker-tasks)
GET | [`/clusters//remove_broker-tasks/`](https://docs.confluent.io/platform/current/kafka-rest/api.html#get--clusters-cluster_id-remove-broker-tasks-broker_id)

## Undocumented features
Verb | URL
--- | ---
POST | [`"/v3/clusters/{clusterId}/topics/{topicName}/records"`](https://github.com/confluentinc/kafka-rest/blob/4ee9931598f8e6579175a8b3a58a40450650714c/kafka-rest/src/main/java/io/confluent/kafkarest/resources/v3/ProduceAction.java#L55)