Add a new admin API getAllLoadReport()

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Motivation

I want to get the load information of all brokers and currently I can only do it by a loop call `org.apache.pulsar.client.admin.BrokerStats#getLoadReport`

Also, I need to save additional IP information for all brokers.

### Solution

Add a new admin API `getAllLoadReport()` to get information about all brokers.

### Alternatives

At the moment I can think of two implementation options:
- Internally call the `getLoadReport` API of all brokers to aggregate their data.
- Refer to the `org.apache.pulsar.testclient.BrokerMonitor` implementation to get information from `ZooKeeper`.
https://github.com/apache/pulsar/blob/002fa49692ff4b8b038bf2edeeb3b32ed218e64f/pulsar-testclient/src/main/java/org/apache/pulsar/testclient/BrokerMonitor.java#L51-L55

Any better suggestions?

### Anything else?

_No response_

### Are you willing to submit a PR?

- [X] I'm willing to submit a PR!

Start with org.apache.pulsar.client.admin.BrokerStats#getLoadReport and the referenced pulsar-testclient/src/main/java/org/apache/pulsar/testclient/BrokerMonitor.java implementation. Compare aggregating broker calls with reading ZooKeeper, then determine the admin API entry point and tests needed; done means getAllLoadReport() returns load information for all brokers, including the required IP information.

Written by the indexing model from the issue text.