Add Scala Native support using libcurl backend

[webhop123]

This PR adds support for Scala Native 0.5.x to requests-scala.

Key Changes:

• Modularized Source: Separated JVM-specific logic into src-jvm/ and created src-native/ for Scala Native.
• Libcurl Backend: Implemented the core HTTP request logic using libcurl via Scala Native's C interop.
• Windows Support: Configured the build to work with vcpkg for dependency management on Windows.
• Compatibility Shims: Added shims for missing java.net and javax.net.ssl classes in Scala Native for Windows.
• Verified: Confirmed GET and POST requests are working on Windows Native, and existing JVM tests still pass.

/claim #156