Improve onboarding performance

[timroes]

What

Fixes #10403

This adds a new web_backend API to determine the onboarding state of a workspace, i.e. return whether it has sources/destinations/connections via one API, so we don't require to call all list APIs separately just to figure out the count.

Also move logic from the OnboardingPage into the individual steps, so that we don't need to load the data required in ANY step upfront before rendering the first step.

This significantly improves page loading of the onboarding step for a new user. There'll now be a (smaller) page load size on each step, for loading the corresponding data needed (e.g. the source definitions in the source step, which were earlier all loaded upfront).