feat(spanner): support unified credentials

[devbww]

Fixes #6464.

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This change is 

[google-cloud-cpp-bot]

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For commit: d88fc35919935e0a9c70319691b44dcc647e0ace

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[codecov]

Codecov Report

Merging #7824 (dcc837c) into main (1a8817c) will increase coverage by 0.00%.
The diff coverage is 99.49%.

@@           Coverage Diff            @@
##             main    #7824    +/-   ##
========================================
  Coverage   95.17%   95.18%            
========================================
  Files        1280     1283     +3     
  Lines      115463   115845   +382     
========================================
+ Hits       109895   110270   +375     
- Misses       5568     5575     +7     

Impacted Files	Coverage Δ
google/cloud/spanner/internal/spanner_stub.h	100.00% <ø> (ø)
google/cloud/spanner/internal/spanner_auth.h	75.00% <75.00%> (ø)
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	83.63% <100.00%> (+0.03%)	⬆️
google/cloud/spanner/client.cc	98.16% <100.00%> (+0.01%)	⬆️
...anner/integration_tests/client_integration_test.cc	96.30% <100.00%> (+0.06%)	⬆️
...integration_tests/session_pool_integration_test.cc	100.00% <100.00%> (ø)
google/cloud/spanner/internal/connection_impl.cc	94.88% <100.00%> (ø)
...gle/cloud/spanner/internal/connection_impl_test.cc	97.69% <100.00%> (+<0.01%)	⬆️
google/cloud/spanner/internal/spanner_auth.cc	100.00% <100.00%> (ø)
google/cloud/spanner/internal/spanner_auth_test.cc	100.00% <100.00%> (ø)
... and 8 more

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: dcc837c26e226dac14298fa7ab6662f0ae384ea1

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[coryan]

Thanks.