src/cpp-common: pass user message iterator class to `UserSourceComponent` and `UserFi...

src/cpp-common: pass user message iterator class to `UserSourceComponent` and `UserFilterComponent`

 🍎 Pass the user message iterator class type as template parameters to
    `UserSourceComponent` and `UserFilterComponent`.
 🍎 Make them set a public type `MessageIterator` with that.
 🍎 Update the `BT_CPP_PLUGIN_*` macros to not take the message iterator
    class type, but use `_userComponentClass::MessageIterator`
    instead.

Change-Id: I7af3a1a088a61e5b83e4a4047f808435221352cc
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11800
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>