Provide API to register Embedded Experience Org Extensions

Currently, the only organization extension that can be managed (create, update, delete) via REST API is Community Apps (widgets). We will need to have the same type of API for all organization extension types, namely *Activity Stream's Embedded Experiences*.

REASON:
This is needed to automate all necessary configurations during customer onboarding. Otherwise, the customer has to do this manually, which is tedious, complicated, highly error prone, not scalable, and ultimately is poor SaaS perception to a customer (given rest of process is automated).