[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/Cargo.lock

@@ -625,6 +625,8 @@ dependencies = [
  "pin-project-lite",
  "rustversion",
  "serde",
+ "serde_json",
+ "serde_path_to_error",
  "sync_wrapper",
  "tokio",
  "tower",
@@ -1000,11 +1002,13 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "app_test_support",
+ "axum",
  "base64",
  "chrono",
  "codex-app-server-protocol",
  "codex-arg0",
  "codex-backend-client",
+ "codex-chatgpt",
  "codex-common",
  "codex-core",
  "codex-feedback",
@@ -1018,6 +1022,7 @@ dependencies = [
  "mcp-types",
  "os_info",
  "pretty_assertions",
+ "rmcp",
  "serde",
  "serde_json",
  "serial_test",