Make project out extensible

[3noch]

This allows power users to extend exeConfigurable and get modified versions of server and linuxExe, etc. that reflect their modifications.

I have:

• [x] Based work on latest develop branch
• [x] Looked for lint in my changes with hlint . (lint found code you did not write can be left alone)
• [x] Run the test suite: $(nix-build -A selftest --no-out-link)
• [ ] (Optional) Run CI tests locally: nix-build release.nix -A build.x86_64-linux --no-out-link (or x86_64-darwin on macOS)

[madeline-os]

I have converted this to a draft while we work out some finer details. There are attributes of server and linuxExe that are not extensible, such as version, and we do not want to confuse users who venture to override them. Once we have a clean separation of attributes that obelisk must control, and attributes that the user may override, we can revisit this.