src.ctf.*: add `ctf::src::NullCpFinder`

src.ctf.*: add `ctf::src::NullCpFinder`

An instance of this new class keeps a temporary code unit of length
`CodeUnitLenV` bytes to help finder a null codepoint (U+0000) in
contiguous buffers (especially when the encoded codepoint spans more
than one buffer).

See the comment of the new class to learn more.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I80feb4402cc0256d7c48c357ca23e355c4322ce0
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12707