src/cpp-common: add `LogCfg` class

src/cpp-common: add `LogCfg` class

`LogCfg` is a class to carry around the logging information used by
`BT_LOG*()` macros in the in-tree plugins.

Philippe added:

* Class comment.

* Setting other members when possible (for example, `_mSelfComp` when
  using the self message iterator constructor).

* Use references instead of pointers in constructors because they can't
  be `nullptr`, making it possible to remove some assertions.

* Component class member (because the libbabeltrace2 API has nothing
  to get a self component class from a self component) and
  compClsName() method.

  compClsName() is always available when using the self message
  iterator, component, or component class constructor.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ic6980a2bcf74f8b270b0a108efc79f25f0e65d8d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7936
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10808
Tested-by: jenkins <jenkins@lttng.org>