cli: move plugin loading code to its own file

cli: move plugin loading code to its own file

The following patch will need to call load and access plugins from a
different file (within the CLI).  Move everything that manages the
plugins in the CLI to its own file, to make it easier to call those
functions in separate files.

Change-Id: I7f62b554066ff7605bd6e1a8cdccd3d8d468ca2e
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1643
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>