[transactions] Implement KIP-664 DescribeProducers

[gaoran10]

This PR needs #1985.

Motivation

Implement KIP-664 DescribeProducers.

Modifications

Handle the command DESCRIBE_PRODUCERS.

Documentation

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[codecov]

Codecov Report

Merging #1986 (d60d786) into master (e931b6d) will decrease coverage by 0.50%.
Report is 2 commits behind head on master.
The diff coverage is 0.20%.

@@             Coverage Diff              @@
##             master    #1986      +/-   ##
============================================
- Coverage     17.72%   17.22%   -0.50%     
  Complexity      751      751              
============================================
  Files           195      195              
  Lines         14156    14568     +412     
  Branches       1322     1352      +30     
============================================
+ Hits           2509     2510       +1     
- Misses        11464    11875     +411     
  Partials        183      183              

Files Changed	Coverage Δ
...streamnative/pulsar/handlers/kop/DelayedFetch.java	0.00% <ø> (ø)
...ative/pulsar/handlers/kop/KafkaCommandDecoder.java	0.31% <0.00%> (-0.02%)	⬇️
...ative/pulsar/handlers/kop/KafkaRequestHandler.java	0.94% <0.00%> (-0.12%)	⬇️
...ndlers/kop/coordinator/group/GroupCoordinator.java	0.00% <0.00%> (ø)
...oordinator/transaction/TransactionCoordinator.java	0.00% <0.00%> (ø)
.../kop/coordinator/transaction/TransactionState.java	0.00% <0.00%> (ø)
...ordinator/transaction/TransactionStateManager.java	0.00% <0.00%> (ø)
...sar/handlers/kop/storage/AppendRecordsContext.java	0.00% <0.00%> (ø)
...sar/handlers/kop/storage/ProducerStateManager.java	0.00% <0.00%> (ø)
...ve/pulsar/handlers/kop/storage/ReplicaManager.java	0.00% <0.00%> (ø)
... and 3 more

... and 7 files with indirect coverage changes