Improve support for custom OpenSSL selection

[masa-koz]

Description

This pull request introduces improved support for building with an external OpenSSL installation, primarily by adding a new openssl_external feature and enhancing the build system to handle more flexible OpenSSL configuration. The changes affect CMake configuration, Rust feature flags, and build scripts to better support both static and shared linking, and to allow specifying OpenSSL paths via environment variables.

Build system and CMake improvements:

• Added a new CMake cache variable QUIC_OPENSSL_ROOT_DIR to allow specifying the OpenSSL root directory, and updated OpenSSL selection logic to support this variable for both static and shared builds. [1] [2]
• Enhanced the CMake logic to properly select between static and shared OpenSSL libraries based on the BUILD_SHARED_LIBS flag, and improved handling of imported OpenSSL targets to avoid duplicate dependency linking.

Rust feature and build script updates:

• Introduced a new Cargo feature openssl_external for building with an external OpenSSL, and updated the build script (scripts/build.rs) to configure CMake accordingly by passing the appropriate variables from environment variables if set. [1] [2]
• Adjusted the output directory logic in the build script to use lib on Windows and artifacts elsewhere, improving compatibility with different platforms.

Test and platform compatibility:

• Updated test logic in src/rs/config.rs to recognize the new openssl_external feature when determining which TLS provider is active on Windows.

Testing

No

Documentation

TBD

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.86%. Comparing base (45a46f3) to head (a9bf108).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5848      +/-   ##
==========================================
- Coverage   86.30%   84.86%   -1.45%     
==========================================
  Files          60       60              
  Lines       18732    18732              
==========================================
- Hits        16167    15897     -270     
- Misses       2565     2835     +270     

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[guhetier]

As a general comment, this start to be a lot of options to handle OpenSSL.
I would be interesting to find a way to keep the complexity from getting out of hands, although I don't have a solution in mind, and it doesn't have to be in this PR.

[masa-koz]

@guhetier I'm sorry for my late response. I addressed your comments.

[masa-koz]

@guhetier I've fixed the format. Please make rerun CI.

[masa-koz]

@guhetier What should I do before merge?