Fix function postgres_gin_pending_list_size

This updated version of the function handles partitioned indexes and renames its output field to correctly indicate the units as bytes, not pages.

Details are in the comments of the SQL files in this commit.

This function change supports a corresponding gitlab-exporters MR: gitlab-cookbooks/gitlab-exporters!278 (merged) which will slow issue: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/14386