feat(agentregistry): generate library

[scotthart]

PiperOrigin-RevId: 933175272

[gemini-code-assist]

Code Review

This pull request introduces the new Agent Registry API C++ client library, including the necessary configuration files, build system support (Bazel and CMake), and documentation. The review comments identified that the quickstart configuration files were using C++14 instead of the required C++17 standard and pointed out an unnecessary copy in the quickstart code that should be optimized to use a reference.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.23%. Comparing base (82a1287) to head (b3f0e89).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16208      +/-   ##
==========================================
- Coverage   92.24%   92.23%   -0.01%     
==========================================
  Files        2265     2265              
  Lines      210126   210126              
==========================================
- Hits       193834   193819      -15     
- Misses      16292    16307      +15     

☔ 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.