Add support for choosing between native-tls and rustls-tls backends
through feature flags, with native-tls as the default for maximum
platform compatibility.
Key changes:
- Add mutually exclusive native-tls and rustls-tls feature flags
- Use conditional compilation to select TLS implementation
- Configure rustls-tls with platform certificate verifier
- Refactor to workspace-based dependency management
- Update CI workflows with improved cross-compilation support
- Add comprehensive TLS backend documentation
The native-tls backend uses system TLS libraries (OpenSSL on Linux,
Secure Transport on macOS, SChannel on Windows) while rustls-tls
provides a pure Rust implementation with platform certificate stores.
