PATCH: Add hooks for pg_total_relation_size and pg_indexes_size

Hello PostgreSQL Hackers,

I am submitting a patch to add hooks for the functions
pg_total_relation_size and pg_indexes_size. These hooks allow for custom
behaviour to be injected into these functions, which can be useful for
extensions and other custom PostgreSQL modifications.

Patch details:

- Adds pg_total_relation_size_hook and pg_indexes_size_hook
- Modifies pg_total_relation_size and pg_indexes_size to call these
hooks if they are set
- Adds necessary type definitions and extern declarations

This feature is useful because it allows for more flexible and customizable
behaviour in relation size calculations, which can be particularly valuable
for extensions that need to account for additional storage outside of the
standard PostgreSQL mechanisms.

The patch is attached.

Thank you for considering this patch. I look forward to your feedback.

Kind regards,
Abdoulaye Ba