Re: Extension security improvement: Add support for extensions with an owned schema

Attached is an updated version of this patch that fixes a few issues
that CI reported (autoconf, compiler warnings and broken docs).

I also think I changed the pg_upgrade to do the correct thing, but I'm
not sure how to test this (even manually). Because part of it would
only be relevant once we support upgrading from PG18. So for now the
upgrade_code I haven't actually run.