TMF: Add an interface for strings regarding a session configuration

TMF: Add an interface for strings regarding a session configuration

Creation of the lttng2.core.session package with the strings that will be used
for session configuration generation. We also externalized the bundle for the
kernel and ust plugin.

Change-Id: If51a4805064ed701a2bbf31a7f327406410f71a6
Signed-off-by: Guilliano Molaire <guilliamo-jaime.molaire@polymtl.ca>
Reviewed-on: https://git.eclipse.org/r/23422
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>