feat: add configurable TLS backend selection with native-tls as default (#1541)

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.

oauth/examples/oauth_async.rs

@@ -42,7 +42,7 @@ async fn main() {
     {
         Ok(client) => client,
         Err(err) => {
-            eprintln!("Unable to build an OAuth client: {}", err);
+            eprintln!("Unable to build an OAuth client: {err}");
             return;
         }
     };