feat(bigtable): add Clear(std::string&) helper for RowKeyType compatibility

[drona-gyawali]

This pr solve the issue #15248

Adds Clear(std::string& key) function that calls std::string::clear() in OSS builds. Google-land will have corresponding overload for absl::Cord::Clear() to replace deprecated absl::Cord::clear().

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This change is 

[scotthart]

/gcbrun

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 92.96%. Comparing base (0423bfa) to head (d027ba7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15252   +/-   ##
=======================================
  Coverage   92.96%   92.96%           
=======================================
  Files        2402     2402           
  Lines      217556   217556           
=======================================
+ Hits       202244   202251    +7     
+ Misses      15312    15305    -7     

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[scotthart]

/gcbrun

[scotthart]

/gcbrun

[scotthart]

/gcbrun