Switch to using the new `KafkaAdminClient` where appropriate

[jeffwidman]

Some of the code in this utility could be simplified by leveraging kafka-python's new KafkaAdminClient, for example the code for fetching consumer group offsets is very straightforward if you use https://kafka-python.readthedocs.io/en/master/apidoc/KafkaAdminClient.html#kafka.admin.KafkaAdminClient.list_consumer_group_offsets.

Note that this is a new class so the interfaces are a little unstable, particularly the ones that still return raw structs... but the ones that return more pythonically formatted data should generally be stable.