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

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

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

The two new available types are `bt2::ClockClass` and
`bt2::ConstClockClass`.

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

Because `bt2::ClockClass::userAttributes()` needs to access the
libbabeltrace2 pointer of the map value, `bt2::CommonValue` makes
`bt2::ClockClass` a friend.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I40e012e1682208f2a43896be3a952cdf8ae0889b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4602