Add the server kind to the Nexus API context

[bnaecker]

• Adds a server kind enum, used to distinguish which API server is running any particular handler.
• Wraps the existing ServerContext into a higher-level ApiContext type, which includes the former in addition to the kind of server handling the request.
• Fixes Check for remote address in allow-list update should ignore underlay #5735

[bnaecker]

The proximal reason for doing this is #5735 , to elide the check when setting the source IP allow-list that ensures you can't lock yourself out. That should help in recovery situations.

I'm a bit on the fence about this. It feels like the nuclear option for a nearly trivial fix (adding a check on the server kind in the allow-list update handler). While it does make the server kind available in an API handler, that has to be passed in as an argument to any other function which needs it. That's nice and explicit, but is a bit counter to the shared state we've got smeared around Nexus in most other situations.

Still, it does solve the original problem, and might be useful in the future for code that needs to know more about which server is handling any particular request.

[jgallagher]

I get why you're on the fence with this (and thank you for doing such a monotonous set of changes to do this), but I think I'm on board. I would not be offended if you want to get a second or third opinion, though! :)

[jgallagher]

Did we need to implement Borrow? It's a little off-the-beaten-path and I thought was more about Cow / ToOwned and being used as a key in collections, none of which seems like it'd be relevant for ApiContext.

I could see an argument for implementing Deref, though, to allow calling ServerContext methods directly on an ApiContext; I wonder if that would have made some of the other mechanical changes unnecessary? I probably wouldn't go back and do this, though - Deref can be nice but can also be a little surprising.

[bnaecker]

We do need borrow here, to satisfy this trait bound:

omicron/nexus/db-queries/src/authn/external/mod.rs

Lines 49 to 54 in 8c38ad1

	pub async fn authn_request<Q>(
	&self,
	rqctx: &dropshot::RequestContext<Q>,
	) -> Result<authn::Context, authn::Error>
	where
	Q: Borrow<T> + Send + Sync + 'static,

I agree that it seems a bit weird, and I'm not sure I like the trait bound itself. I kept it simply to minimize churn, but I'm happy to reevaluate.

I also considered implementing Deref. I think it might make some other changes unnecessary, but there actually didn't seem to be many ServerContext methods used themselves. Most of the fields are visible and used directly. I also agree it's a bit magical sometimes, so I think it makes sense to defer it.

[jgallagher]

TIL! I'd definitely defer any reevaluation of that for some followup work (which is probably not necessary at all - implementing Borrow is surprising but actually required - that's fine).

[bnaecker]

Whoops, I've linked to the wrong issue here. This fixes #5735. I'll amend the commit message to reflect that now.