cpp-common/bt2: field.hpp: Add assignment operator for Enum field

cpp-common/bt2: field.hpp: Add assignment operator for Enum field

Used to set the value of enumeration fields using integers.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Iaec161ca8e9e0d13770ac4f82a9c5556a1a20001
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8199
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10834
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>