feat: Implement internal CursorClient

[dpcollins-google]

This will be used by the kafka shim.

[codecov]

Codecov Report

Merging #252 into master will increase coverage by 0.05%.
The diff coverage is 76.92%.

@@             Coverage Diff              @@
##             master     #252      +/-   ##
============================================
+ Coverage     72.74%   72.80%   +0.05%     
- Complexity      759      773      +14     
============================================
  Files           135      138       +3     
  Lines          4033     4085      +52     
  Branches        213      213              
============================================
+ Hits           2934     2974      +40     
- Misses          975      986      +11     
- Partials        124      125       +1     

Impacted Files	Coverage Δ	Complexity Δ
...google/cloud/pubsublite/internal/CursorClient.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...loud/pubsublite/internal/CursorClientSettings.java	75.00% <75.00%> (ø)	3.00 <3.00> (?)
...le/cloud/pubsublite/internal/CursorClientImpl.java	79.06% <79.06%> (ø)	10.00 <10.00> (?)

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