feat(rest): enable PQC EC curves in libcurl

[scotthart]

The errors that can result when a call to set CURLOPT_SSL_EC_CURVES are not recoverable during program execution: CURLE_UNKNOWN_OPTION, CURLE_SSL_ENGINE_SETFAILED, CURLE_NOT_BUILT_IN, CURLE_BAD_FUNCTION_ARGUMENT, CURLE_OUT_OF_MEMORY.

Thus, we can check once when the rest client is created and avoid log spam.

[gemini-code-assist]

Code Review

This pull request configures post-quantum cryptography (PQC) curves via CURLOPT_SSL_EC_CURVES when using libcurl 7.73.0 or later, and adds corresponding unit tests. The feedback suggests changing the log level from INFO to DEBUG when the option is unsupported or fails to set, which prevents log pollution and performance overhead on every request.

[codecov]

Codecov Report

❌ Patch coverage is 76.19048% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.26%. Comparing base (a52ae45) to head (0e49452).

Files with missing lines	Patch %	Lines
google/cloud/internal/curl_impl_test.cc	80.95%	4 Missing ⚠️
google/cloud/internal/curl_rest_client.cc	62.50%	3 Missing ⚠️
google/cloud/internal/curl_rest_client_test.cc	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16243      +/-   ##
==========================================
- Coverage   92.27%   92.26%   -0.01%     
==========================================
  Files        2214     2214              
  Lines      206394   206436      +42     
==========================================
+ Hits       190443   190470      +27     
- Misses      15951    15966      +15     

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