feat(spanner): add support for lock_hint enum

[mpeddada1]

This PR does the following:

• Implements the LockHint enum.
• Implements Option type for the enum.
• Allows it to be set via ReadParams in conn->Read.
  The setup is very similar to what we have for feat(spanner): add support for order_by enum #15240

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 96.46018% with 4 lines in your changes missing coverage. Please review.

Project coverage is 92.93%. Comparing base (65a6980) to head (3b7578c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/spanner/internal/connection_impl.cc	72.72%	3 Missing ⚠️
...gle/cloud/spanner/internal/connection_impl_test.cc	97.91%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #15249    +/-   ##
========================================
  Coverage   92.92%   92.93%            
========================================
  Files        2394     2394            
  Lines      215461   215564   +103     
========================================
+ Hits       200227   200329   +102     
- Misses      15234    15235     +1     

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[scotthart]

Reviewed 9 of 11 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: 10 of 11 files reviewed, 1 unresolved discussion

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google/cloud/spanner/client.cc line 57 at r2 (raw file):

  auto directed_read_option = ExtractOpt<DirectedReadOption>(opts);
  internal::OptionsSpan span(std::move(opts));
  auto lock_hint = ExtractOpt<LockHintOption>(opts);

Here and in the other places we call ExtractOpt for the LockHintOption, we want to do this before we create the OptionSpan.

[mpeddada1]

Here and in the other places we call ExtractOpt for the LockHintOption, we want to do this before we create the OptionSpan.

Ah good catch, thank you! Done, also added additional testing in client_test.

[scotthart]

Reviewed 2 of 2 files at r3, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @mpeddada1)