KAFKA-19362: Finalize homogeneous simple share assignor

[AndrewJSchofield]

Finalise the share group SimpleAssignor for homogeneous subscriptions. The assignor code is much more accurate about the number of partitions assigned to each member, and the number of members assigned for each partition. It eliminates the idea of hash-based assignment because that has been shown to the unhelpful. The revised code is very much more effective at assigning evenly as the number of members grows and shrinks over time.

A future PR will address the code for heterogeneous subscriptions.

Reviewers: Apoorv Mittal [email protected]