cpp-common/bt2: add `bt2::SelfMessageIterator`

cpp-common/bt2: add `bt2::SelfMessageIterator`

This patch introduces `bt2::SelfMessageIterator` which wraps
`bt_self_message_iterator`.

This is the last piece of C++ binding needed to write a complete
component class in C++, immediately wrapping the component class and
message iterator class methods parameters.

`bt2::SelfMessageIterator` offers:

createMessageIterator():
    Create a message iterator from this message iterator:

        auto msgIter = selfMsgIter.createMessageIterator(
                           selfMsgIter.component().inputPorts()["in"]);

component():
    Borrow the parent self component.

    This is usually used to retrieve some component data:

        selfMsgIter.component().data<MyComponentData>();

port():
    Borrow the output port from which this message iterator operates.

isInterrupted():
    Whether or not this message iterator is interrupted.

data():
    Get and set user data.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I02747ffb7a31743a310bdeff120f1d1042bf11e6
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11237
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Simon Marchi <simon.marchi@efficios.com>