bt2c/logging.hpp: use `std::span<const std::uint8_t>` for mem. data

bt2c/logging.hpp: use `std::span<const std::uint8_t>` for mem. data

`std::span<const std::uint8_t>` is also the `bt2c::Logger::MemData`
alias.

Adapt `src/plugins/ctf/common/src/msg-iter/msg-iter.cpp` accordingly.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I553c0bbb0fb532bd23506a9760155371820e773d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12456
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>