cpp-common/bt2: fix trivial-ish mistakes in plugin-dev.hpp

cpp-common/bt2: fix trivial-ish mistakes in plugin-dev.hpp

Fix a few problems that were found by exercising the `plugin-dev.hpp`
code a bit more.

    ⚾ Use public inheritance in `SinkCompClsBridge`.
    ⚾ Add return statement in happy path of
       `SinkCompClsBridge::graphIsConfigured`.
    ⚾ Use `DataT&` as the parameter type to `_addOutputPort` and
       `_addInputPort` methods, matching what
       `Self*Component::add*Port` expects.
    ⚾ Call `_outputPortConnected` instead of `outputPortConnected` in
       `UserSourceComponent::outputPortConnected`, which otherwise results
       in infinite recursion.

Change-Id: Ieab12436c4b85b5a33195be0ea388b3f545f07cd
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11789
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>