[1/2] make client-side-versioned APIs versioned

[iliana]

Part of #8922. Makes bootstrap-agent, installinator, nexus-internal, and repo-depot versioned such that any incompatible changes are flagged.

Instead of the usual instructions for creating a new version, a strategically-positioned comment which references #9290 tells developers not to create any new versions. This comment will be visible in diffs if any versions are added.

Note that this does not add the version policy to the Dropshot listeners yet, because in currently-deployed systems the version in the header is 0.0.1. I think this should work okay during an upgrade as long as the policy isn't in place on the server side. I'll have a second PR up to add the version policy shortly, and we should merge that after systems currently performing Nexus-driven updates (only dogfood, I think?) are updated after this lands.

[davepacheco]

I'd love @sunshowers's eyes on this as well to make sure I'm not missing something!

[iliana]

1. I don't think this is an R17 blocker so I've removed the milestone.
2. I'll also update this to add the version policy to the APIs, and fix the related tests. I think I know enough now to be certain that clients sending the 0.0.1 header will still hit the 1.0.0 APIs.
3. We discussed having the clients point to the hashed, versioned OpenAPI document instead of the latest symlink. I'll do that as well.