Addition of alias types regpublication and regsubscription

Hi,

I felt inclusion of alias types regpublication and regsubscription will
help the logical replication users. This will also help in [1].
The alias types allow simplified lookup of publication oid values for
objects. For example, to examine the pg_publication_rel rows, one could
write:
SELECT prpubid::regpublication, prrelid::regclass FROM pg_publication_rel;

rather than:
SELECT p.pubname, prrelid::regclass FROM pg_publication_rel pr,
pg_publication p WHERE pr.prpubid = p.oid;

Similarly in case of subscription:
For example, to examine the pg_subscription_rel rows, one could write:
SELECT srsubid::regsubscription, srrelid::regclass FROM pg_subscription_rel;

rather than:
SELECT s.subname,srsubid::regclass FROM pg_subscription_rel sr,
pg_subscription s where sr.srsubid = s.oid;

Attached patch has the changes for the same.
Thoughts?

[1] -
https://www.postgresql.org/message-id/flat/CALDaNm0OANxuJ6RXqwZsM1MSY4s19nuH3734j4a72etDwvBETQ%40mail.gmail.com

Regards,
Vignesh