Refactor onboarding concerns around onboarding

What does this MR do and why?

Refactor onboarding concerns around onboarding into more of a facade pattern

• when using at controller and helper layer across multiple controllers, we need to centralize a bit.

📓 this is the final MR in a series of MRs for this feature driven from #417609 (closed)

How to set up and validate locally

note all paths in this MR covered and can be followed in ee/spec/features/registrations/saas

How to travel down one of these paths and land in onboarding -> !109243 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #417609 (closed)