lib: add bt_field_class_variant_borrow_option_by_{index,name}()

Those are non-const versions of the existing
bt_field_class_variant_borrow_option_by_index_const() and
bt_field_class_variant_borrow_option_by_name_const().

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I322c202c35d815012d2f6de494dce76a161692e4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1920
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>