[feature request] Support Consumer.getLastMessageId

First off, thank you for all the development you do on Pulsar! :smile:

### Motivation

When combining data from two Consumers by event timestamps and the topics have significant backlog, it gets tricky to make sure that all relevant events have arrived. If we could check from the Consumer whether we have already read the latest message, that would simplify the code.

The C++ client already supports Consumer.getLastMessageIdAsync. (Though the Java client has deprecated the singular version of that method, supposedly due to the Consumers being able to subscribe to several topics concurrently. For us right now the singular version is enough.)

Start by inspecting the Consumer API in the pulsar-client-node repository and the existing C++ client's getLastMessageIdAsync support. Add the requested Consumer method for the Node client, then verify that it reports whether the latest message has been read, including coverage for consumers with significant backlog.

Written by the indexing model from the issue text.