Use `ensure_unique_id` in runner-related tables to use a trigger for the id assignment

The following discussion from !182549 (merged) should be addressed:

• @tianwenchen started a discussion:

  @pedropombeiro should we use ensure_unique_id to change to use a trigger for the id assignment like the other partitioned table?

  If we don't do that, since the partitioned table ci_runners's id column's default is NULL, the new partition will have default value as NULL instead of nextval('ci_runners_id_seq'::regclass).

  But I guess this can be done in a follow-up since this shouldn't be a problem for existing partitions.