feat: Implement PubsubLiteConsumer.

[dpcollins-google]

This implements Kafka's Consumer API.

[codecov]

Codecov Report

Merging #287 into master will increase coverage by 9.50%.
The diff coverage is 61.24%.

@@             Coverage Diff              @@
##             master     #287      +/-   ##
============================================
+ Coverage     61.68%   71.19%   +9.50%     
+ Complexity      984      892      -92     
============================================
  Files           189      151      -38     
  Lines          6063     4735    -1328     
  Branches        525      236     -289     
============================================
- Hits           3740     3371     -369     
+ Misses         2017     1230     -787     
+ Partials        306      134     -172     

Impacted Files	Coverage Δ	Complexity Δ
...oogle/cloud/pubsublite/kafka/ConsumerSettings.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...gle/cloud/pubsublite/kafka/PubsubLiteConsumer.java	70.85% <70.85%> (ø)	53.00 <53.00> (?)
...blite/internal/AutoValue_CursorClientSettings.java
...l/wire/AutoOneOf_ConnectedSubscriber_Response.java
...ite/beam/AutoValue_TopicBacklogReaderSettings.java
...le/cloud/pubsublite/AutoValue_PublishMetadata.java
...loud/pubsublite/AutoValue_AdminClientSettings.java
...teUnboundedReader_PartitionedSequencedMessage.java
...com/google/cloud/pubsublite/AutoValue_Message.java
...m/google/cloud/pubsublite/beam/AutoValue_Uuid.java
... and 35 more

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[manuelmenzella-google]

Minor nits. Thanks.