[RFC] trappy: add support for event callbacks

Created by: joelagnel

This series adds event callbacks support to trappy so that trappy users can register callbacks for trace events, that can be used to drive a state machine to do some analysis.

Example analysis script using this is here: https://github.com/joelagnel/lisa/blob/40047eab82ed43152ef9d587ba89c5145cdb5d5b/get_wlat_cbs.py (disclaimer, the example script in the above link is very wip and needs to cleaned up and polished properly)