lib: bt_field_class_dynamic_array_create(): accept length FC parameter

This patch makes bt_field_class_dynamic_array_create() similar to
bt_field_class_variant_with_selector_create() for consistency: it
accepts the length field class on creation instead of setting it
afterwards.

You can pass `NULL` to create a dynamic array field class without a
length field class.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib94adcc32128154b5979120ef0623be912f28734
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1718
Tested-by: jenkins <jenkins@lttng.org>