feat: Add the TopicStats client

[palmere-google]

Adds an interface and implementation for the TopicStatsClient. I've kept
them both in internal for now since we don't yet expose an API for
interacting with the CursorService.

[codecov]

Codecov Report

Merging #179 into master will decrease coverage by 0.18%.
The diff coverage is 68.04%.

@@             Coverage Diff              @@
##             master     #179      +/-   ##
============================================
- Coverage     64.21%   64.02%   -0.19%     
- Complexity      643      650       +7     
============================================
  Files           128      133       +5     
  Lines          3831     3886      +55     
  Branches        198      198              
============================================
+ Hits           2460     2488      +28     
- Misses         1260     1287      +27     
  Partials        111      111              

Impacted Files	Coverage Δ	Complexity Δ
...m/google/cloud/pubsublite/AdminClientSettings.java	0.00% <0.00%> (-34.79%)	0.00 <0.00> (-1.00)
...le/cloud/pubsublite/internal/TopicStatsClient.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../pubsublite/internal/TopicStatsClientSettings.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...loud/pubsublite/internal/TopicStatsClientImpl.java	70.00% <70.00%> (ø)	7.00 <7.00> (?)
...gle/cloud/pubsublite/internal/AdminClientImpl.java	90.90% <91.66%> (-0.86%)	24.00 <10.00> (-4.00)
...in/java/com/google/cloud/pubsublite/Constants.java	100.00% <100.00%> (ø)	1.00 <1.00> (?)
...loud/pubsublite/internal/RetryingExecutorUtil.java	100.00% <100.00%> (ø)	4.00 <4.00> (?)
... and 2 more

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[dpcollins-google]

Mostly lgtm. make the requested changes (most of them surrounding code reuse) then ping me again before merging if you could.

trying to remove approval

[palmere-google]

PTAL

[dpcollins-google]

LGTM, please fix the one nit?