TMF XML: Add an extension point to let plugins advertise builtin XML files

TMF XML: Add an extension point to let plugins advertise builtin XML files

Plugins will be able to pre-package some XML-defined analysis and views using
this extension point.

Change-Id: I433b91c76b513ce3298d4ef7d3e6a9ec6fd0890d
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/23211
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>