Introduce onboarding status to ee

What does this MR do and why?

Introduce onboarding status to ee. A MVC from larger !122532 (merged)

• centralization of onboarding concerns
• extend facade pattern from foss
• see #417609 (closed) for more info

note :: prefixed on a lot of ee Onboarding calls due to loading issues so that the top level instantiation is found instead of the newly introduced EE::Onboarding module that is for prepending foss code.

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)