Feat: add cross-zone headers (routing region + zone) for grpc

[pulkit4tech]

• Description and context for reviewers: one partner, one stranger

RELEASE NOTES:

• add cross-zone reserved headers (routing region + zone) for grpc
• update transportRequestToMetadata to skip these headers (avoid reaching them in handler)
• exempt routing header for isReserved check

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.65%. Comparing base (b739f41) to head (d8dd9f2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2390      +/-   ##
==========================================
- Coverage   84.69%   84.65%   -0.04%     
==========================================
  Files         276      276              
  Lines       16117    16122       +5     
==========================================
- Hits        13650    13648       -2     
- Misses       2021     2024       +3     
- Partials      446      450       +4     

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