nix: Provide lib in flakes with buildPgxExtension

[Hoverbear]

The currently pgx extension build derivations are quite non-trivial, and are right now provisioned as part of the nix flake init -t github:zombodb/pgx command.

If we were to find ways to improve it, users would have to manually update, which is un-ideal.

This PR creates a number of lib functions in our crate, including one that builds pgx extensions.

Once this is landed in the main branch, users could do:

mkdir pgx-demo
cd pgx-demo
nix init -t github:zombodb/pgx
chmod +x .cargo/pgx-linker-script.sh
cargo fetch
nix flake check