[connectors] Support connectors part 1 - App server & MCP (#9667)

In order to make Codex work with connectors, we add a built-in gateway
MCP that acts as a transparent proxy between the client and the
connectors. The gateway MCP collects actions that are accessible to the
user and sends them down to the user, when a connector action is chosen
to be called, the client invokes the action through the gateway MCP as
well.

 - [x] Add the system built-in gateway MCP to list and run connectors.
 - [x] Add the app server methods and protocol

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -1,5 +1,6 @@
 mod account;
 mod analytics;
+mod app_list;
 mod collaboration_mode_list;
 mod config_rpc;
 mod initialize;