feat: add native-tls as an optional TLS backend (#631)

Add reqwest-native-tls feature flag to allow users to choose between
rustls (default) and native-tls for HTTP transports.

native-tls uses platform-native TLS implementations:
- OpenSSL on Linux
- Secure Transport on macOS
- SChannel on Windows

This is particularly useful for Linux distribution packagers who need
to link against system TLS libraries (e.g., OpenSSL) rather than
bundling a separate TLS implementation. Linking against system libs
ensures security updates are applied system-wide and satisfies
distribution packaging policies.

Updated documentation to explain the available TLS backend options.

crates/rmcp/Cargo.toml

@@ -88,6 +88,8 @@ reqwest = ["__reqwest", "reqwest?/rustls-tls"]
 
 reqwest-tls-no-provider = ["__reqwest", "reqwest?/rustls-tls-no-provider"]
 
+reqwest-native-tls = ["__reqwest", "reqwest?/native-tls"]
+
 server-side-http = [
   "uuid",
   "dep:rand",