nixos/nix-daemon: Organize buildMachine options with a submodule

[Valodim]

This PR picks up #47636. It changes the nix.buildMachine option from an attribute set to a submodule, with documentations and examples per option.

A couple of thoughts:

1. The sshKey option is a secret path, which there is no good solution for so far. See Add types.secretPath #78640
2. There's no assertion to check for presence of both system and systems at the same time, because I couldn't figure out how to do assertions per module nicely :) I guess it's an artifact anyways that there are even two options for this, I'd rather deprecate systems, and type system as str or listOf str, expecting the former to cover basically all cases.
3. As per this suggestion, I put the features variables an enum, and added an "extra" option that allows freeform features. The idea is to have a somewhat curated list of commonly used names where typos can't happen, and move special ones to their own option.

Ttested this locally, works as expected :)

[Valodim]

@msteen thanks for the review :) fixed all your points.

I also changed system and systems to default to builtins.currentSystem if neither are set, that seems to be a reasonable default.

[msteen]

Seems good to me now!

[Ericson2314]

@Valodim thanks for picking this up for me! And @infinisil, thanks for letting me know.

[Ericson2314]

Can you add a 20.09 change log breaking change notice?

[Valodim]

I updated this to incorporate all feedback (and rebased on master).

I think we actually ended up compatible with (sane usage of) the previous definitions, at least the config implementation looks identical modulo handling of defaults to me, and the example from the previous documentation produces identical results.

There is a slight potential for errors in configs that used sloppy types before, e.g. a string value for maxJobs will lead to an error like this:

error: The option value 'nix.buildMachines.[definition 1-entry 1].maxJobs' in ... is not of type 'signed integer'.
I added an item to the release notes about that.