Bigtable: expose credential provider as a toplevel setting

[igorbernstein2]

Extracted from #4498

[codecov]

Codecov Report

Merging #4508 into master will decrease coverage by 0.3%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #4508      +/-   ##
============================================
- Coverage     48.83%   48.53%   -0.31%     
+ Complexity    21932    20742    -1190     
============================================
  Files          2077     2077              
  Lines        207078   203046    -4032     
  Branches      23407    23294     -113     
============================================
- Hits         101119    98539    -2580     
+ Misses        97120    96431     -689     
+ Partials       8839     8076     -763

Impacted Files	Coverage Δ	Complexity Δ
...gtable/admin/v2/BigtableInstanceAdminSettings.java	100% <100%> (ø)	6 <1> (+1)	⬆️
.../bigtable/admin/v2/BigtableTableAdminSettings.java	71.73% <100%> (+2.69%)	7 <1> (+1)	⬆️
...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%)
...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%)
...gle/cloud/storage/testing/RemoteStorageHelper.java	55.37% <0%> (-2.36%)	8% <0%> (ø)
... and 467 more

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