Create syscache entries for pg_extension

Two syscache identifiers are added for extension names and OIDs.

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

Another case where this is useful is a loaded library where some of its
code paths rely on some objects of the extension to exist; it can be
simpler and more efficient to do an existence check directly on the
extension through the syscache.

Author: Jelte Fennema-Nio
Reviewed-by: Alexander Korotkov, Pavel Stehule
Discussion: https://postgr.es/m/CAGECzQTWm9sex719Hptbq4j56hBGUti7J9OWjeMobQ1ccRok9w@mail.gmail.com

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