impl(bigtable): add DeadlineOption

[scotthart]

This PR adds the bigtable::DeadlineOption for specifying the deadline for a single attempt at an RPC. This new option is being introduced first in Bigtable but will eventually be expanded to apply to other services (#4926).

[gemini-code-assist]

Code Review

This pull request introduces a Bigtable-specific MergeOptions function to handle a new DeadlineOption, which allows setting per-RPC attempt deadlines by wrapping GrpcSetupOption. The generator and Bigtable clients are updated to use this new logic. Feedback points out missing header includes in the new options.h and options.cc files. Additionally, there is a concern regarding performance inefficiencies caused by redundant nested lambda wrapping when options are merged at multiple levels.

[codecov]

Codecov Report

❌ Patch coverage is 96.06299% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.70%. Comparing base (1b9b3c1) to head (a91f29f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/bigtable/options_test.cc	95.50%	4 Missing ⚠️
google/cloud/bigtable/options.cc	95.83%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16085    +/-   ##
========================================
  Coverage   92.70%   92.70%            
========================================
  Files        2351     2353     +2     
  Lines      217954   218068   +114     
========================================
+ Hits       202047   202167   +120     
+ Misses      15907    15901     -6     

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