feat(storage): Respect custom endpoint for SignedUrl

[bajajneha27]

closes #13068

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This change is 

[coryan]

I manually started the GHA builds and added you to the magic list that can start such jobs.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.25926% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92.94%. Comparing base (45ed21b) to head (f69266a).

Files	Patch %	Lines
google/cloud/storage/client.cc	90.00%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #14179    +/-   ##
========================================
  Coverage   92.94%   92.94%            
========================================
  Files        2048     2048            
  Lines      180801   180917   +116     
========================================
+ Hits       168039   168154   +115     
- Misses      12762    12763     +1     

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[bajajneha27]

We can make some small improvements to make the Request classes immutable (or less mutable). Your call. I am happy either way.

By this, did you mean passing the endpoint_authority to the constructor?