bt2: rename `_Field.field_class` -> `_Field.cls`

bt2: rename `_Field.field_class` -> `_Field.cls`

This makes it match what we have for the class of an event object, the
class of a stream object, and the class of a trace object (`cls`
properties).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I5dd6f837a7f79b05ca933cd30bd281da2fbe5728
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1919
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>