Add group coordinator lookup

[jeffwidman]

Add group coordinator lookup

We need a way to send a request to the group coordinator.

I spent a day and a half trying to implement a _send_request_to_group_coordinator()
that included:

1. caching the value of the group coordinator so that it wouldn't have
   to be repeatedly looked up on every call. This is particularly important
   because the list_consumer_groups(), list_consumer_group_offsets(),
   and describe_consumer_groups() will frequently be used by monitoring
   scripts. I know across the production clusters that I support, using a
   cached value will save ~1M calls per day.
2. clean and consistent error handling. This is difficult because the
   responses are inconsistent about error codes. Some have a top-level
   error code, some bury it within the description of the actual item.
3. Avoiding tight coupling between this method and the request/response
   classes... the custom parsing logic for errors etc, given that it's
   non-standard, should live in the callers, not here.

So finally I gave up and just went with this simpler solution and made
it so the callers can optionally bypass this if they somehow already
know the group coordinator.

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This change is 

[dpkp]

I believe that v1 adds the idea of a group type. All groups I have seen are consumers, but I think Kafka Streams may use group coordination for producers or even other abstract groups. In any event, we'd need to add a parameter for group type I believe.

[jeffwidman]

Good thought.

I'm going to merge this for now and will add that as part of switching it to a FindCoordinatorRequest (which is a bigger change of renaming things across the client/consumer code). Added it to #1630 (comment)

[tvoinarovskyi]

Emm, TransactionCoordinator is looked up by that too)