[Project Ekala] Add reverse-proxy build tool for distributed nix builds

[jonringer]

Traditionally you would have a "beefy hydra" instance where that client could schedule across many remote hosts. However, it would be great for there to be a "remote host" which is able to distribute builds transparently.

This would be highly valuable to projects, CI tooling, and businesses.

Somewhat overlaps with what nix-build.net tries to achieve. But would like an open-source self-hostable version.