Implement ResourceLoader.getClusterForTopic(topic: KafkaTopic): KafkaCluster | undefined

Implement `ResourceLoader` method to conveniently get at the KafkaCluster instance given one of its topics.

When completed, patch up `resourceScaffoldProjectRequest()`'s determination of topic -> cluster, plus probably any remaining callers to `ResourceManager..getClusterForTopic()` (and the kill that old API off entirely).

Any caller to `ResourceManager.getClusterForTopic()` is vulnerable to failure if calling with a direct-connection-based KafkaTopic, hence the bug qualification.

Should be pretty simple:
1. Validate topic's connection id matches the loader's.
2. Call this.getKafkaClustersForEnvironmentId(topic.environmentId)
3. Find + return the one matching topic.clusterId, otherwise undefined.

Semi-related, if `resourceScaffoldProjectRequest()` cannot find kafka cluster for the topic, it should raise a Sentry error for this unexpected condition using `logError()`. Then we would have known if happening in the wild.