KAFKA-16730: Initial version of share group consumer client code

[AndrewJSchofield]

This is the initial version of the share group consumer client code. It implements the complete ShareConsumer interface.

There are unit tests, but not integration tests yet since those would depend upon complete broker code, which is not available at this point.

Committer Checklist (excluded from commit message)

• [ ] Verify design and implementation
• [ ] Verify test coverage and CI build status
• [ ] Verify documentation (including upgrade notes)