.github/CODEOWNERS: add myself as the PostgreSQL owner

Since the development of #38698, I have been radically overhauling the
PostgreSQL support and plan on maintaining it in the long run, taking
over from @ocharles, who authored most of it initially.

However, while #38698 has not been merged (yet), I'm taking the time to
go ahead and add myself as the owner of all the related code, so that I
can be marked as a reviewer for things people submit in the mean time.

Signed-off-by: Austin Seipp <aseipp@pobox.com>