feat(pubsublite): routing publisher implementation

[tmdiep]

partitionCountWatcher polls for updates to the number of topic partitions.

routingPublisher routes messages to multiple singlePartitionPublishers and handles the topic partition count increasing. Decreases will terminate the routingPublisher.

The initial set of partition publishers must successfully start before the routingPublisher accepts published messages. However, when new partition publishers are added later, messages may be routed to them while they are starting up. The messages are queued in the publishMessageBatcher and sent to the server when the Publish stream connects.

The entire routingPublisher tree terminates if the partitionCountWatcher or any of the singlePartitionPublishers terminate.

[tmdiep]

WANT_LGTM=@manuelmenzella-google, @palmere-google
Other reviewers optional, but comments welcome.

If anyone feels strongly about waiting for the new partition publishers to successfully start before routing messages to them, feel free to let me know. It can be done, but introduces some complexity, so I went with this. If any partition publisher fails to start up, the entire routing publisher will be terminated anyway.