[MCP] Introduce an experimental official rust sdk based mcp client (#4252)

The [official Rust
SDK](57fc428c57)
has come a long way since we first started our mcp client implementation
5 months ago and, today, it is much more complete than our own
stdio-only implementation.

This PR introduces a new config flag `experimental_use_rmcp_client`
which will use a new mcp client powered by the sdk instead of our own.

To keep this PR simple, I've only implemented the same stdio MCP
functionality that we had but will expand on it with future PRs.

---------

Co-authored-by: pakrym-oai <pakrym@openai.com>

codex-rs/core/src/codex.rs

@@ -377,7 +377,10 @@ impl Session {
         // - load history metadata
         let rollout_fut = RolloutRecorder::new(&config, rollout_params);
 
-        let mcp_fut = McpConnectionManager::new(config.mcp_servers.clone());
+        let mcp_fut = McpConnectionManager::new(
+            config.mcp_servers.clone(),
+            config.use_experimental_use_rmcp_client,
+        );
         let default_shell_fut = shell::default_user_shell();
         let history_meta_fut = crate::message_history::history_metadata(&config);