Native Nix store implementation / standardized REST API

[roberth]

Both cachix and attic implement their own push-related endpoints, that are, for good reason, incompatible with Nix's own HTTP binary cache store. This is not so great for interoperability.

What does the attic API look like? Is it compatible with the cachix API? Would you be interested in working towards a native HttpBinaryCacheStore subclass in Nix?

Some benefits of a store implementation:

• No need to install an extra package in the CIs like GitHub Actions
• Automatic support in deployment tools that use nix copy
• Support for queries like nix path-info --store