impl(rest): use PQC algorithm when available

[scotthart]

This PR queries the underlying crypto library and configures libcurl to use a hybrid MLKEM algorithm if available. It also includes other non-PQC algorithms as a fallback in case the server does not support the same algorithm as the client.

This PR also introduces a new showcase build that spins up a gapic-showcase server to verify the hybrid MLKEM algorithm is being used.

[gemini-code-assist]

Code Review

This pull request introduces support for Post-Quantum Cryptography (PQC) curves (specifically X25519MLKEM768) in the REST transport client by configuring CURLOPT_SSL_EC_CURVES in libcurl when supported. It adds platform-specific implementations to query available crypto groups on OpenSSL and Windows, along with integration tests to verify PQC negotiation. The review feedback identifies a critical buffer overflow in the Windows implementation of AvailableCryptoGroups, a potential null pointer dereference in the OpenSSL implementation, a missing <algorithm> header, and opportunities to optimize string copies and use more idiomatic move semantics.

[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.26%. Comparing base (ce70c7d) to head (c8da154).

Files with missing lines	Patch %	Lines
google/cloud/internal/curl_rest_client.cc	71.42%	2 Missing ⚠️
google/cloud/internal/ssl_ec_curves_test.cc	96.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16253   +/-   ##
=======================================
  Coverage   92.26%   92.26%           
=======================================
  Files        2214     2217    +3     
  Lines      206307   206398   +91     
=======================================
+ Hits       190355   190440   +85     
- Misses      15952    15958    +6     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.