Improve support for custom OpenSSL selection

[masa-koz]

Description

This pull request improves support for using external OpenSSL installations with the build system, especially for static and shared library builds. It introduces a new Cargo feature, better environment variable handling, and more robust CMake logic for finding and linking OpenSSL. The changes are grouped by theme below.

Build system improvements for external OpenSSL:

• Added a new openssl-external Cargo feature to enable building with an external OpenSSL installation. (Cargo.toml Cargo.tomlR93)
• Updated the Rust build script (scripts/build.rs) to handle the new feature, passing appropriate CMake definitions and reading OpenSSL paths from environment variables (OPENSSL_DIR, OPENSSL_INCLUDE_DIR, OPENSSL_LIB_DIR). (scripts/build.rs scripts/build.rsR51-R64)
• Changed the output directory for the native MsQuic library in the build script to use an artifacts subdirectory for better organization. (scripts/build.rs scripts/build.rsL32-R32)

CMake configuration enhancements:

• Added a QUIC_OPENSSL_DIR CMake cache variable to specify the path to an OpenSSL installation, complementing the existing include and lib directory options. (CMakeLists.txt CMakeLists.txtR115)
• Improved CMake logic for finding and linking OpenSSL libraries: now distinguishes between static and shared builds, uses the correct library suffix, and avoids duplicate dependency linking by only linking the main OpenSSL libraries at this stage. (CMakeLists.txt CMakeLists.txtR776-R807)

Testing

No

Documentation

TBD