cpp-common/bt2: move component class bridges to `internal/comp-cls-bridge.hpp`

cpp-common/bt2: move component class bridges to `internal/comp-cls-bridge.hpp`

The bridges are going to be used when creating component classes at
runtime, not only in the context of plugins.

Change-Id: If6d04531ee660692d1a6eb84950a584530a23566
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11813
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>