sarama/v2 should not leak its internals to the caller

[dnwe]

The Broker type used internally by Sarama is also directly used in the MetadataResponse type to decode the Broker id, host and (if applicable) rack into. This internal type then leaks further into the admin.go functionality such as admin.DescribeCluster()

For Sarama/v2 we should ensure that the protocol types are distinct from any internal structs, and ideally the admin.go interfaces should also have their own dedicated types for what is returned to the enduser.