[nix-cache] Add cache copy command

[mikeland73]

Summary

Note: this may not be the final UX for this command. I'm just getting it in hidden so we can build the cache feature end-to-end and will refine details later.

Adds devbox cache copy <url> command. It copies the current devbox nix profile to the url. <url> can be anything supported by https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-copy

There are two future plans for this command:

• url will be optional when logged in. It will be determined by API in those cases. We may want to move url to be a flag in that case (possibly the --to flag to mimic nix copy).
• This command will support specifying the package you wish to copy. Packages can be nix installable, but also devbox packages (i.e. package@version). We could even support runx in the future.

I used copy to mimic nix command name, but I slightly prefer upload. That and the above changes would make this command look like:

devbox cache upload [--to <url>] [package]

or as simple as

devbox cache upload

to upload current profile when logged in.

How was it tested?

devbox cache copy "s3://mike-test-nix-cache?region=us-west-2"

[savil]

The PR description plan makes sense. I like the suggested changes.

[savil]

The long and the short mean quite different things. Which is it?

[savil]

Could both be: "Copies all nix packages in current project to the cache at <url>"?