Draft: WIP build against postgresql 18-devel

Work towards postgresql-18 support, which we want to test for io performance. This branch doesn't compile because postgres changed the signature of several of the functions in their extensions API. Several look like they're trivial (adding a new parameter that can be NULL to preserve old behavior) but their change to tuple descriptors will take a little more work to support. I'm just leaving this MR here so the branch doesn't get lost