feat: add CommitTimestamp support

[devjgm]

Part of #1217, #1218, #1219

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This change is 

[coryan]

Assuming the builds pass, this Thanks!

Reviewed 4 of 4 files at r1, 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #1225 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1225      +/-   ##
==========================================
+ Coverage   94.84%   94.84%   +<.01%     
==========================================
  Files         185      185              
  Lines       13766    13812      +46     
==========================================
+ Hits        13056    13100      +44     
- Misses        710      712       +2

Impacted Files	Coverage Δ
google/cloud/spanner/value.cc	97.46% <100%> (+0.2%)	⬆️
google/cloud/spanner/value_test.cc	99.13% <100%> (+0.04%)	⬆️
google/cloud/spanner/value.h	92.8% <100%> (+0.1%)	⬆️
google/cloud/spanner/timestamp.h	100% <100%> (ø)	⬆️
...loud/spanner/internal/partial_result_set_resume.cc	90.9% <0%> (-4.55%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	85.71% <0%> (-2.07%)	⬇️
...anner/integration_tests/client_integration_test.cc	98.47% <0%> (-0.02%)	⬇️
google/cloud/spanner/keys.h	100% <0%> (ø)	⬆️
google/cloud/spanner/internal/transaction_impl.h	100% <0%> (ø)	⬆️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	92.36% <0%> (ø)	⬆️
... and 2 more

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