feat: Implement Consumer.endOffsets

[tmdiep]

Retrieves head cursors to implement PubsubLiteConsumer.endOffsets.

[codecov]

Codecov Report

Merging #102 (98c9964) into master (8ca9b61) will increase coverage by 0.30%.
The diff coverage is 70.83%.

@@             Coverage Diff              @@
##             master     #102      +/-   ##
============================================
+ Coverage     66.82%   67.13%   +0.30%     
- Complexity      127      131       +4     
============================================
  Files            10       10              
  Lines           621      642      +21     
  Branches         22       23       +1     
============================================
+ Hits            415      431      +16     
- Misses          196      201       +5     
  Partials         10       10              

Impacted Files	Coverage Δ	Complexity Δ
...oogle/cloud/pubsublite/kafka/ConsumerSettings.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...gle/cloud/pubsublite/kafka/PubsubLiteConsumer.java	76.76% <80.95%> (+0.63%)	61.00 <4.00> (+4.00)

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