JSON Impl and schema for BuildResult

[Ericson2314]

Motivation

More JSON!

CC @mschwaig @edef1c it occurs to me that perhaps BuildResult was (at least closer to) the type for build trace values we wanted all along. E.g. being able to sign build failures is cool, and if we move the signature here, we also sign for all the outputs at once as @mschwaig wanted.

Context

Depends on #14425

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[edolstra]

FWIW Determinate Nix also has a JSON serialization for BuildResult: https://github.com/DeterminateSystems/nix-src/blob/236ebef6514f3a2a9765c8a1d80dd503b8e672be/src/libstore/build-result.cc. This is used to provide structured JSON log messages containing the build result. Might be worth having a look at that to avoid a compatibility issue.

One difference is that the status strings follow the enum values, i.e. ResolvesToAlreadyValid instead of resolves to already valid. I feel the use of whitespace falsely suggests that these are arbitrary human-readable strings rather than enum values.

[Ericson2314]

That looks like the only difference? I'm happy to go back to camel case. I had a similar feeling after I did it that way too.

edit this is now done.

[Ericson2314]

OK This is now converted to camel case, matching the det sys serialization.

[dpulls]

🎉 All dependencies have been resolved !