This adds PostgreSQL 16 as a vendored postgresql version, and adapts the
code to support this version.
The important changes to PostgreSQL 16 compared to the PostgreSQL 15
changeset include the addition of a neon_rmgr instead of altering Postgres's
original WAL format.
Co-authored-by: Alexander Bayandin <alexander@neon.tech>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>