Refactor login endpoints, instrument console endpoints

[david-crespo]

Pulling these refactors out of #7339 because they're mechanical and just add noise. The point is to make it a cleaner diff when we add the function calls or wrapper code that creates audit log entries, as well as to clean up the device_auth (eliminated) and console_api (shrunken substantially) files, which have always been a little out of place.

Refactors

With the change to a trait-based Dropshot API, the already weird console_api and device_auth modules became even weirder, because the actual endpoint definitions were moved out of those files and into http_entrypoints.rs, but they still called functions that lived in the other files. These functions were redundant and had signatures more or less identical to the endpoint handlers. That's the main reason we lose 90 lines here.

Before we had

http_entrypoints.rs -> console_api/device_auth -> nexus/src/app functions

Now we (mostly) cut out the middleman:

http_entrypoints.rs -> nexus/src/app functions

Some of what was in the middle moved up into the endpoint handlers, some moved "down" into the nexus "service layer" functions.

One (1) functional change

The one functional change is that the console endpoints are all instrumented now.

[david-crespo]

I still don't like this method, but at least it's a few lines shorter now

[david-crespo]

All this function did was call service_console_index. That's the kind of thing we're fixing here.

[david-crespo]

Some of these are slightly more interesting than inlining a function call. This helper function did all the stuff you normally see here at top level in the endpoint handlers (instrumentation call, pulling stuff out of path and query params, etc), and the nitty gritty datastore logic that normally goes inside nexus/src/app. So I pulled the former out to here and the latter into nexus/src/app/login.rs.

The idea was to make these look more like the other handlers.

[david-crespo]

I created these to reduce boilerplate back when the actual endpoint handlers were defined here (rather that silly little helper functions called by the endpoint handlers, which is what they are now). Because of the change to Dropshot API, we kinda have to have the boilerplate over in http_entrypoints.rs anyway, so these are pointless. Deleting this section is why we now inline console_index_or_login_redirect(rqctx).await calls in a ton of handlers.