lib: Add functions to borrow specialized component classes from specialized components

The API currently contains a function to obtain a 'bt_component_class *'
from a 'bt_component *'.  This patch adds functions to obtain a
specialized component class from a specialized component.  For example,
a 'bt_component_class_source *' from a 'bt_component_source *'.

Change-Id: Ide192c7759f630adbc884def2e5c462ce2b8f973
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1161
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>