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 both session configuration generation and LTTng analysis requirements.
We also externalized the package for the kernel and ust plugin.

Change-Id: I78030f04b937f30d6ed89e08a5b8d8b9c1c64379
Signed-off-by: Guilliano Molaire <guilliamo-jaime.molaire@polymtl.ca>