bt2: factor out some code in field class creation

bt2: factor out some code in field class creation

Remove `_TraceClass._check_field_class_create_status()` and
`_TraceClass._set_field_class_user_attrs()`.  Add
`_TraceClass._check_and_wrap_field_class()` and use it.  The new method
combines the behavior of the two removed ones, plus wrapping the SWIG
pointer with the correct Python type (found using
`bt2_field_class._obj_type_from_field_class_ptr()`).

Change-Id: Iaa4871acb7bad4d21baae537802c20353ed25f22
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13399
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>