mTLS REST prototype

[scotthart]

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 47.91667% with 125 lines in your changes missing coverage. Please review.

Project coverage is 92.85%. Comparing base (97d29c3) to head (a173342).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/tests/mtls_object_basic_crud_integration_test.cc	6.52%	86 Missing ⚠️
google/cloud/internal/curl_impl.cc	17.39%	19 Missing ⚠️
google/cloud/internal/unified_rest_credentials.cc	0.00%	9 Missing ⚠️
google/cloud/internal/unified_grpc_credentials.cc	0.00%	5 Missing ⚠️
...cloud/storage/internal/unified_rest_credentials.cc	0.00%	3 Missing ⚠️
google/cloud/internal/oauth2_mtls_credentials.h	50.00%	2 Missing ⚠️
google/cloud/mtls_credentials_config.cc	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15054      +/-   ##
==========================================
- Coverage   92.90%   92.85%   -0.05%     
==========================================
  Files        2351     2357       +6     
  Lines      210211   210451     +240     
==========================================
+ Hits       195296   195414     +118     
- Misses      14915    15037     +122     

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