Min and max aggregates for UUIDs

Hi,

With the support for UUIDv7 in PostgreSQL 18 it also makes sense to
support aggregates like `min` and `max` for UUIDs.

For example for queries like these:

SELECT bookshop_id, uuid_extract_timestamp(max(id)) AS
latest_book_created FROM books GROUP BY bookshop_id;

In the meantime I use custom aggregates but it would be great to have
these builtin.

BTW many thanks for the great UUIDv7 support and all the fantastic work
on PostgreSQL!

Cheers, Rudolph