[ML] Add loading indicator for job capabilities on wizard open

[pheyos]

Follow-up on #77326

During the job capabilities field loading, an empty page is displayed. In case this loading takes a bit longer, it's not clear to the user that something is happening, this might even look like an error.

Suggestion: Add a loading indicator while the job capabilities are fetched (AD + DFA wizards).

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[jgowdyelastic]

The work here would be to add a loading indicator for all pages during the routing resolve stage of the page rendering.
This was originally planning when the work was done to remove the angular router, but because of the speed of the page loading and because the original angular router didn't do this either, it was deemed low priority.