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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