Add support for rack id

[ditas]

Allow to provide rack id for kpro_req_lib:fetch/9 function

[zmstone]

Please document the new config option for the start_link API
https://github.com/kafka4beam/brod/pull/543/files#diff-bda931b0049203cab6d623157a37dea600ec29fce6fa9aaf2720cf25d5bca176R224

[zmstone]

Suggested change

	kpro_req_lib:fetch(Vsn, Topic, Partition, Offset,
	#{ max_wait_time => WaitTime
	, min_bytes => MinBytes
	, max_bytes => MaxBytes
	, isolation_level => IsolationLevel
	, rack_id => RackId
	}).
	kpro_req_lib:fetch(Vsn, Topic, Partition, Offset,
	#{ max_wait_time => WaitTime
	, min_bytes => MinBytes
	, max_bytes => MaxBytes
	, isolation_level => IsolationLevel
	, rack_id => RackId
	}).

[zmstone]

Hi @ditas
Thank you for the PR.
I'm not quite familiar with the rack_id feature from Kafka.
Does it imply that you can fetch from any broker in the cluster?
If that's the case, it might not be very useful here because brod_consumer always fetches from the partition leader.

[zmstone]

fetch/8 should call fetch/9 (with ?undef as the last arg).