RFC: full API support for hosted and self-managed versions of the stack

[JoshMock]

An RFC is needed to identify the best solution for supporting multiple stack versions—Serverless, plus actively supported minor versions of hosted and self-managed—in the CLI.

Requirements

• Publish initial draft of RFC
• Get first round of reviews from teammates
• Follow up with any needed revisions or polish, and document the chosen solution

[JoshMock]

Internal RFC document published to GDocs; currently iterating through a first round of review with @flobernd.