Implement pgrx-based Packaging on *nix platforms

[theory]

Create a crate that uses the design found in #51 to build trunk packages for pgrx distributions on macOS, Linux, and FreeBSD (and any other Unix-like platforms supported by the #45 repo). Add it to the CLI as described in the “Trunk Packaging Commands” specified in the CLI Sketch (e.g., pgxn trunk build).

Done when:

• Crate released
• Updated CLI released
• Works for all pgrx projects in the Design source registry repository with metadata patches to merge #45 repo for *nix platforms