Changes

tmf: Add configuration source extension point and classes

Introduction of "org.eclipse.tracecompass.tmf.core.config" extension
point. Extensions need to implement the interface
"ITmfConfigurationSource". The "TmfConfigurationSourceManager" will
read all extensions and interested parties can get all configuration
source types implementing"ITmfConfigurationSourceType" and their
"ITmfConfigurationSource". Using the "ITmfConfigurationSource"
instances of "ITmfConfiguration" can be instantiated and managed, e.g.
updated or removed.

This commit also contains relevant JUnit tests of the new classes.

[Added] org.eclipse.tracecompass.tmf.core.config extension point

Change-Id: Iff1935178515cafab015bbcad3ddc7a519e63f8c
Signed-off-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/c/tracecompass/org.eclipse.tracecompass/+/204455
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Trace Compass Bot <tracecompass-bot@eclipse.org>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>