[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.)

まず、pulsar-client-node リポジトリの Consumer API と、既存の C++ クライアントにおける getLastMessageIdAsync のサポートを調査します。次に、Node クライアントに要求された Consumer メソッドを追加し、最新のメッセージが読み取られたかどうかを報告することを検証します。大幅なバックログがある consumer も対象に含めてください。

索引モデルが issue の本文から書いたものです。