src/cpp-common: make `bt2c::JsonVal` visitable

src/cpp-common: make `bt2c::JsonVal` visitable

This patch adds the bt2c::JsonVal::accept() method which accepts a JSON
value visitor (`bt2c::JsonValVisitor`), dispatching to the virtual
bt2c::JsonVal::_accept() method.

This is part of an effort to support CTF2‑SPEC‑2.0 [1].

[1]: https://diamon.org/ctf/CTF2-SPEC-2.0.html

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I75462799f9433ee868b618ecba4ba63b5822b96c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7485
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12685