[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/core/config.schema.json

@@ -153,6 +153,9 @@
             "collaboration_modes": {
               "type": "boolean"
             },
+            "connectors": {
+              "type": "boolean"
+            },
             "elevated_windows_sandbox": {
               "type": "boolean"
             },
@@ -1135,6 +1138,9 @@
         "collaboration_modes": {
           "type": "boolean"
         },
+        "connectors": {
+          "type": "boolean"
+        },
         "elevated_windows_sandbox": {
           "type": "boolean"
         },