Add database role to SpannerConfig for role-based access control.

[hmehrotra]

Please add a meaningful description for your change here
Propagate database role for fine-grained access control in Cloud Spanner.

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[ahmedabu98]

LGTM

[hmehrotra]

R: @pabloem

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Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[darshan-sj]

LGTM

[codecov]

Codecov Report

Merging #23377 (b05fb88) into master (30dbe09) will increase coverage by 0.00%.
The diff coverage is 80.64%.

❗ Current head b05fb88 differs from pull request most recent head d642f04. Consider uploading reports for the commit d642f04 to get more accurate results

@@ Coverage Diff @@## master #23377 +/- ##
========================================
Coverage 73.45% 73.46% ========================================
Files 718 718 Lines 95770 95935 +165 ========================================
+ Hits 70352 70475 +123 - Misses 24107 24149 +42 
Partials 1311 1311 

Flag	Coverage Δ
python	83.21% <83.73%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/runtime/xlangx/expand.go	0.00% <0.00%> (ø)
sdks/python/apache_beam/ml/inference/base.py	95.65% <ø> (-0.19%)	⬇️
...thon/apache_beam/ml/inference/pytorch_inference.py	0.00% <0.00%> (ø)
...hon/apache_beam/ml/inference/tensorrt_inference.py	0.00% <0.00%> (ø)
sdks/python/apache_beam/coders/coders.py	87.36% <65.21%> (-0.65%)	⬇️
sdks/python/apache_beam/typehints/schemas.py	91.89% <76.08%> (-1.96%)	⬇️
...on/apache_beam/runners/dataflow/dataflow_runner.py	82.91% <83.33%> (-0.09%)	⬇️
...ks/python/apache_beam/runners/worker/statecache.py	92.93% <94.66%> (+1.06%)	⬆️
sdks/python/apache_beam/coders/coder_impl.py	93.94% <100.00%> (+0.13%)	⬆️
...ks/python/apache_beam/coders/coders_test_common.py	98.58% <100.00%> (+0.03%)	⬆️
... and 17 more

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[pabloem]

lgtm. I'll merge once we get clean tests.