Create syscaches for pg_extension

Shared libraries of extensions might want to invalidate or update their
own caches whenever a CREATE/ALTER/DROP EXTENSION command is run for
their extension (in any backend). Right now this is non-trivial to do
correctly and efficiently. But if the extension catalog was part of a
syscache this could simply be done by registering an callback using
CacheRegisterSyscacheCallback for the relevant syscache.

This change is only made to make the lives of extension authors easier.
The performance impact of this change should be negligible, since
updates to pg_extension are very rare.