mavlink & system: add events interface

- sending protocol
- uorb event message & template methods for argument packing
- libevents submodule to send common events and handle json files
- cmake maintains a list of all (PX4) source files for the current build
  (PX4 modules + libs), which is used to extract event metadata and
  generate a json file

msg/tools/uorb_rtps_message_ids.yaml

@@ -341,6 +341,8 @@ rtps:
     id: 158
   - msg: estimator_event_flags
     id: 159
+  - msg: event
+    id: 160
   ########## multi topics: begin ##########
   - msg: actuator_controls_0
     id: 180