cpp-common/bt2: add `findPlugin()`

cpp-common/bt2: add `findPlugin()`

Add the `findPlugin()` function, allowing one to find a plugin by name.
The default parameter values used are the same as those used in the
Python bindings.

Change-Id: I659b5b0f0ae18965bbcacdac3c4e0c146c7348e4
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11823
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>