feat(spanner): support for the PG.JSONB data type

[devbww]

Introduce google::cloud::spanner::JsonB for PostgreSQL databases. The interface is the same as Json (just a strongly-typed string), but JsonB values are stored in the database in a decomposed, binary format, which affects performance: JsonB is slower to input, but faster to process as it avoids reparsing.

Note: Support for arrays of JSONB will come later.

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This change is 

[google-cloud-cpp-bot]

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For commit: 17f7b78be409f9b8451afddd111302dab7b70d4d

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[codecov]

Codecov Report

Base: 94.14% // Head: 94.08% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (17f7b78) compared to base (6228499).
Patch coverage: 57.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10098      +/-   ##
==========================================
- Coverage   94.14%   94.08%   -0.06%     
==========================================
  Files        1511     1511              
  Lines      139757   139960     +203     
==========================================
+ Hits       131569   131679     +110     
- Misses       8188     8281      +93     

Impacted Files	Coverage Δ
google/cloud/spanner/samples/postgresql_samples.cc	0.72% <0.00%> (-0.06%)	⬇️
...cloud/spanner/testing/database_integration_test.cc	78.50% <ø> (ø)
...r/integration_tests/data_types_integration_test.cc	74.24% <22.41%> (-11.94%)	⬇️
google/cloud/spanner/value.cc	90.67% <90.90%> (-0.02%)	⬇️
...gle/cloud/spanner/internal/connection_impl_test.cc	97.79% <96.00%> (-0.03%)	⬇️
google/cloud/spanner/value_test.cc	98.42% <97.91%> (-0.03%)	⬇️
google/cloud/spanner/json.h	100.00% <100.00%> (ø)
google/cloud/spanner/value.h	93.75% <100.00%> (+0.03%)	⬆️
...loud/spanner/internal/logging_result_set_reader.cc	88.00% <0.00%> (-4.00%)	⬇️
...e/cloud/spanner/testing/cleanup_stale_instances.cc	75.40% <0.00%> (-3.28%)	⬇️
... and 2 more

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[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• google/cloud/spanner/samples/postgresql_samples.cc:693, tag spanner_postgresql_jsonb_add_column
• google/cloud/spanner/samples/postgresql_samples.cc:713, tag spanner_postgresql_jsonb_update_data
• google/cloud/spanner/samples/postgresql_samples.cc:751, tag spanner_postgresql_jsonb_query_parameter

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