Add C++ interface for the libbabeltrace2 `bt_clock_snapshot` API

Add C++ interface for the libbabeltrace2 `bt_clock_snapshot` API

This patch adds C++ wrappers for Babeltrace 2 clock snapshot objects.

The new available class is `bt2::ClockSnapshot`.

This new class follows the approach of other wrappers in
`src/cpp-common/bt2`.

You can't create such objects: they are what various message methods
(not part of `src/cpp-common/bt2` yet) return.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Id462b88b1575cddf50ae250f22dda71d7b27733c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4603