[feature request] Support Consumer.getLastMessageId

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

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.)

首先检查 pulsar-client-node repository 中的 Consumer API，以及现有 C++ client 对 getLastMessageIdAsync 的支持。然后为 Node client 添加所请求的 Consumer 方法，并验证它是否会报告最新消息是否已被读取，包括覆盖具有大量 backlog 的 consumer。