Sam/nix parallel

[samrose]

What kind of change does this PR introduce?

This draft PR introduces a "passive" experimental parallel build of postgresql 15, 16, experimental orioledb-16 all with known working extensions and wrappers. It will also include gh action workflows that will build and test all these variants, extensions and wrappers. Once those actions are added, I'll switch it from draft to actual PR.

Once this PR is merged, this portion of the project will just update and build in the background, in an attempt to stabilize this work here over time. We won't try to immediately use any of it for build and deployment of production postgres, exts, and wrappers until we see it stabilize here for some time.