impl(gax-internal): host resolution for custom universe domains

[alvarowolfx]

Towards #3646

[codecov]

Codecov Report

❌ Patch coverage is 94.87179% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.73%. Comparing base (e1f7b3c) to head (9e13b1b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gax-internal/src/host.rs	93.93%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5464      +/-   ##
==========================================
- Coverage   97.73%   97.73%   -0.01%     
==========================================
  Files         217      217              
  Lines       48186    48243      +57     
==========================================
+ Hits        47095    47149      +54     
- Misses       1091     1094       +3     

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