Bigtable: Added unit tests for GCRule#fromProto

[rahulKQL]

Fixes #4057

Observation:

If used multiple setXXX() on protobuf GcRule#Builder then the last most added value would be persisted after the build(). This is also valid for model GCRule#fromProto.

Added few unit test cases around the fromProto operation.

[rahulKQL]

@sduskis / @igorbernstein2
Please have a look at the observations, If valid then we can close the mentioned issue.

[igorbernstein2]

The proto uses a one-of for max age & max versions, so you can't set both. This confuses a lot of people, which is one of the reasons we created this wrapper

[codecov]

Codecov Report

Merging #4990 into master will decrease coverage by 0.63%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4990      +/-   ##
============================================
- Coverage     50.32%   49.68%   -0.64%     
+ Complexity    23668    22304    -1364     
============================================
  Files          2238     2238              
  Lines        226059   221344    -4715     
  Branches      24959    24079     -880     
============================================
- Hits         113754   109968    -3786     
+ Misses       103704   102914     -790     
+ Partials       8601     8462     -139

Impacted Files	Coverage Δ	Complexity Δ
...in/java/com/google/cloud/storage/StorageBatch.java	88% <0%> (-4%)	13% <0%> (ø)
...e/cloud/vision/v1p4beta1/ImageAnnotatorClient.java	58.69% <0%> (-3.31%)	15% <0%> (-4%)
...m/google/cloud/vision/v1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
...e/cloud/vision/v1p3beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
...e/cloud/vision/v1p2beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
.../java/com/google/cloud/speech/v1/SpeechClient.java	48.57% <0%> (-2.78%)	10% <0%> (-2%)
...om/google/cloud/speech/v1p1beta1/SpeechClient.java	48.57% <0%> (-2.78%)	10% <0%> (-2%)
...d/webrisk/v1beta1/WebRiskServiceV1Beta1Client.java	63.41% <0%> (-2.5%)	12% <0%> (-3%)
...ogle/cloud/texttospeech/v1/TextToSpeechClient.java	55.88% <0%> (-2.46%)	9% <0%> (-2%)
...cloud/texttospeech/v1beta1/TextToSpeechClient.java	55.88% <0%> (-2.46%)	9% <0%> (-2%)
... and 541 more

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[igorbernstein2]

LGTM