[improve] PIP-307: Use assigned broker URL hints during broker reconnection

[dragosvictor]

Motivation

PIP-307 describes a faster process to reconnect a client to the broker when a topic is unloaded from one broker to another. The server-side implementation is out in 3.2. Client-side support is already checked-in for Java and CPP. This PR adds the respective support to the Golang client. It features improvements to both the producer and consumer, while handling direct and proxied connections alike.

Reference Java and CPP implementations:

• [improve][broker] PIP-307 Added assignedBrokerUrl to CloseProducerCmd to skip lookups upon producer reconnections during unloading pulsar#21408
• [improve][broker] PIP-307: Implement broker and client consumer changes when topic is unloading pulsar#21682
• [improve][broker] PIP-307: Add proxy support for Java client pulsar#21789
• [feat] PIP-307 added assigned broker urls for CloseProudcer and CloseConsumer commands and handler logic pulsar-client-cpp#389

Modifications

• Add optional fields assignedBrokerUrl and assignedBrokerUrlTls to the Protobuf spec of CommandCloseProducer and CommandCloseConsumer
• Forward this information through the client from the source in connection.handleCloseConsumer and connection.handleCloseProducer all the way through to partitionProducer.reconnectToBroker and partitionConsumer.reconnectToBroker, by adding the URLs to the connectionClosed struct
• If the URLs are present, perform exactly one attempt to reconnect via this "assigned" URL (if the connection fails revert to issuing lookup calls on the broker to locate the topic). Leverages the exiting parsing and name resolution mechanism in the Lookup Service. Method lookupService.getBrokerAddress had to be exposed as public method lookupService.GetBrokerAddress for this purpose. Furthermore, had to add method Connection.IsProxied to allow the reconnection to work properly in the context of proxied connections.

Verifying this change

• Make sure that the change passes the CI checks.

Added integration test ExtensibleLoadManagerTestSuite.TestTopicUnload, with subtests for both direct connections and proxied connections. The test requires a different configuration than what we currently have elsewhere, so it runs with its own Docker Compose setup. It launches two brokers having the Extensible Load Manager enabled. The test concurrently:
- Produces messages to a topic
- Consumes messages from the topic
- Unloads the topic to the other broker
At the end, it verifies that messages have been successfully delivered both before and after the unloading, while issuing no further topic lookup calls. On that last point, it relies on the topic lookup metrics to perform the verification, as the test framework doesn't have a straightforward way to mock and intercept calls otherwise. This can be a point for further improvement of the test codebase.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): No
• The public API: No
• The schema: No
• The default values of configurations: No
• The wire protocol: Yes: Added assigned broker URL(s) fields to the producer and consumer close commands.

Documentation

• Does this pull request introduce a new feature? Yes
• If yes, how is the feature documented? Not applicable
• If a feature is not applicable for documentation, explain why? The feature is an internal performance improvement
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation Not applicable