lib: standardize variant field option function names

This patch does the following changes:

bt_field_variant_select_option_field_by_index():
    bt_field_variant_select_option_by_index()

bt_field_variant_get_selected_option_field_index():
    bt_field_variant_get_selected_option_index()

bt_field_variant_borrow_selected_class_option_const():
    bt_field_variant_borrow_selected_option_class_const()

bt_field_variant_with_unsigned_integer_selector_borrow_selected_class_option_const():
    bt_field_variant_with_selector_field_integer_unsigned_borrow_selected_option_class_const()

bt_field_variant_with_signed_integer_selector_borrow_selected_class_option_const():
    bt_field_variant_with_selector_field_integer_signed_borrow_selected_option_class_const()

A variant field is the instance of a variant field class. A variant
field has one or more options which are instances of the variant field
class options. A variant field option contains a field.

The `with_unsigned_integer_selector` to
`with_selector_field_integer_unsigned` and
`with_signed_integer_selector` to `with_selector_field_integer_signed`
renames match what's already in the `bt_field_class` API.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I7c8f19d06d86464b0b2131036c27adb8d909aaa3
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2571
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>