KAFKA-17425: Improve coexistence of consumer and share groups

[AndrewJSchofield]

This PR ensures that using the various group RPCs work properly when issued against the wrong type of group, such as DescribeConsumerGroups for a share group, or ConsumerGroupHeartbeat for a share group. There are no changes to the RPC error codes required.

The significant code changes are:

• Making sure that the group coordinator does not assume that only classic and consumer groups exist. This was the cause of a ClassCastException when ConsumerGroupHeartbeat was being used against a share group.
• Making sure that committing offsets to a share group fails with GroupIdNotFoundException rather than java.lang.UnsupportedOperation. This was the cause of a name collision between a share group and a consumer group when using kafka-consumer-groups.sh --reset-offsets which inadvertently created a consumer group of the same name.

Committer Checklist (excluded from commit message)

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