Add native OAuth 2.0 authentication support to MCP client (#3213)

Co-authored-by: Alex Hancock <alexhancock@block.xyz>

crates/mcp-client/examples/test_auth.rs

@@ -0,0 +1,64 @@
+use anyhow::Result;
+use mcp_client::client::{ClientCapabilities, ClientInfo, McpClient, McpClientTrait};
+use mcp_client::transport::{StreamableHttpTransport, Transport};
+use std::collections::HashMap;
+use std::time::Duration;
+use tracing_subscriber::EnvFilter;
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    // Initialize logging
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            EnvFilter::from_default_env()
+                .add_directive("mcp_client=debug".parse().unwrap())
+                .add_directive("eventsource_client=info".parse().unwrap()),
+        )
+        .init();
+
+    println!("Testing Streamable HTTP transport with OAuth 2.0 authentication...");
+
+    // Create the Streamable HTTP transport for any MCP service that supports OAuth
+    // This example uses a hypothetical MCP endpoint - replace with actual service
+    let mcp_endpoint =
+        std::env::var("MCP_ENDPOINT").unwrap_or_else(|_| "https://example.com/mcp".to_string());
+
+    println!("Using MCP endpoint: {}", mcp_endpoint);
+
+    let transport = StreamableHttpTransport::new(&mcp_endpoint, HashMap::new());
+
+    // Start transport
+    let handle = transport.start().await?;
+
+    // Create client
+    let mut client = McpClient::connect(handle, Duration::from_secs(30)).await?;
+    println!("Client created with Streamable HTTP transport\n");
+
+    // Initialize - this will trigger the OAuth flow if authentication is needed
+    // The implementation now includes:
+    // - RFC 8707 Resource Parameter support for proper token audience binding
+    // - Proper OAuth 2.0 discovery with multiple fallback paths
+    // - Dynamic client registration (RFC 7591)
+    // - PKCE for security (RFC 7636)
+    // - MCP-Protocol-Version header as required by the specification
+    let server_info = client
+        .initialize(
+            ClientInfo {
+                name: "streamable-http-auth-test".into(),
+                version: "1.0.0".into(),
+            },
+            ClientCapabilities::default(),
+        )
+        .await?;
+
+    println!("Connected to server: {server_info:?}\n");
+    println!("OAuth 2.0 authentication test completed successfully!");
+    println!("\nKey improvements implemented:");
+    println!("✓ RFC 8707 Resource Parameter implementation");
+    println!("✓ MCP-Protocol-Version header support");
+    println!("✓ Enhanced OAuth discovery with multiple fallback paths");
+    println!("✓ Proper canonical resource URI generation");
+    println!("✓ Full compliance with MCP Authorization specification");
+
+    Ok(())
+}