add support for an installinator document

[sunshowers]

This document will be fetched by installinator and used to determine which host phase 2 and control plane artifacts to fetch.

Unlike artifacts.json, this document is treated as an opaque blob by wicketd and Nexus (hence no KnownArtifactKind for it).

[iliana]

It seems like we're going through a lot of effort in the Omicron PR to avoid adding the installinator document as a KnownArtifactKind, and I don't really understand why. We don't need to expect that all KnownArtifactKinds are present in a repository, and "KnownArtifactKind" is intended to mean "does control plane software know what to do with this thing?", which Wicket will (and eventually Nexus too I presume).

[iliana]

This really seems like it should be a bool.

[sunshowers]

I personally tend to avoid using bools in public APIs because it can be hard to tell what a particular bool means when looking at code. I don't feel strongly about this though.

[iliana]

I find it makes public APIs that much harder to use because you can't use an existing bool you might have from somewhere else; you have to go through the wordy if blah { LongStructName::Yes } else { LongStructName::No }, made even worse by our company-wide policy on 80-character lines :)

If this were a builder API we'd use a bool because the function would be a lot more obvious. I think it'd be nice to move to a builder eventually instead of continuing to tack on arguments here, but that's not something we need to do in this PR.

[sunshowers]

all right, updated

[labbott]

LGTM