Implement metadata-driven PGXS compilation on Linux & macOS

[theory]

Build a Rust Crate with an interface that can be used by the future CLI to build a PGXS-based distribution with no additional dependencies. Use the meta_spec crate to read the META.json file and to patch it with any metadata missing from the original (or if there is no original).

Should be complete when:

• Five C-based PGXS extensions can be built, installed, and tested
• Works for them all on Linux amd64^ arm64 and macOS on arm64
• A well-documented Rust crate allows does the work