commander: implement external modes and mode executors

msg/RegisterExtComponentRequest.msg

@@ -0,0 +1,17 @@
+# Request to register an external component
+uint64 timestamp # time since system start (microseconds)
+
+uint64 request_id                  # ID, set this to a random value
+char[25] name                      # either the requested mode name, or component name
+
+# Components to be registered
+bool register_arming_check
+bool register_mode                 # registering a mode also requires arming_check to be set
+bool register_mode_executor        # registering an executor also requires a mode to be registered (which is the owned mode by the executor)
+
+bool enable_replace_internal_mode  # set to true if an internal mode should be replaced
+uint8 replace_internal_mode        # vehicle_status::NAVIGATION_STATE_*
+bool activate_mode_immediately     # switch to the registered mode (can only be set in combination with an executor)
+
+
+uint8 ORB_QUEUE_LENGTH = 2