cpp-common/bt2: move user component class bases to `component-class-dev.hpp`

cpp-common/bt2: move user component class bases to `component-class-dev.hpp`

It will become possible to use these classes to create component classes
at runtime, not only in the context of plugins.

Change-Id: I3507a8025fa4e8af6bdb42f2be053dd43f3c2eaa
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11814
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>