Adding first barebone impl for varadhi consumer.

[gauravAshok]

TODO:

• back-pressure from CC
• error handling
• tests
• micro-bench

Future PR:

• optional rate-limiter on RQ/DLQ produce.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 17.13900% with 614 lines in your changes missing coverage. Please review.

Project coverage is 64.62%. Comparing base (78be60c) to head (5334c2d).
Report is 36 commits behind head on master.

Files with missing lines	Patch %	Lines
...art/varadhi/consumer/impl/VaradhiConsumerImpl.java	0.00%	114 Missing ⚠️
...com/flipkart/varadhi/consumer/DelayedConsumer.java	0.00%	83 Missing ⚠️
.../flipkart/varadhi/consumer/MessageSrcSelector.java	0.00%	57 Missing ⚠️
...rt/varadhi/consumer/processing/ProcessingLoop.java	0.00%	52 Missing ⚠️
...dhi/consumer/processing/GroupedProcessingLoop.java	0.00%	51 Missing ⚠️
...i/consumer/processing/UngroupedProcessingLoop.java	0.00%	36 Missing ⚠️
...rt/varadhi/entities/cluster/SubscriptionState.java	10.25%	28 Missing and 7 partials ⚠️
...rt/varadhi/consumer/impl/ConsumersManagerImpl.java	22.50%	31 Missing ⚠️
...m/flipkart/varadhi/entities/InternalQueueType.java	10.71%	25 Missing ⚠️
.../flipkart/varadhi/consumer/concurrent/Context.java	27.27%	14 Missing and 2 partials ⚠️
... and 32 more

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #147      +/-   ##
============================================
+ Coverage     63.11%   64.62%   +1.50%     
- Complexity      543     1120     +577     
============================================
  Files           133      211      +78     
  Lines          2790     5702    +2912     
  Branches        168      376     +208     
============================================
+ Hits           1761     3685    +1924     
- Misses          964     1860     +896     
- Partials         65      157      +92     

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