src/cpp-common/bt2/trace-ir.hpp: add nameSpace() methods where needed

src/cpp-common/bt2/trace-ir.hpp: add nameSpace() methods where needed

The bt2::CommonEventClass::nameSpace() and
bt2::CommonStreamClass::nameSpace() methods make it possible to set and
get the namespace of event classes and stream classes (new feature
starting with MIP 1).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I6bd37d130fe8aaa8180181e41127e1ebfeb1ea4b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8003
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12698