Fixes #4909
### Motivation

It would be good to expose method `getLastMessageId` in `ConsumerImpl` to a public method. 
eg. some times user would like to know the lag messages; or only consume messages before current time.

### Modifications

- expose method `getLastMessageId` in consumer api.
- add unit test.

### Verifying this change
Ut passed

(cherry picked from commit 93d95c74)