Make "alpha" changes possible in the REST API
Problem
Our GraphQL API supports adding schema as alpha
but our REST API does not.
Adding items as alpha allows us to be certain of an addition, measure its performance, and dogfood it before we commit to supporting it when we release it.
Our REST API does not allow breaking changes within v4, so we must be careful about what is added to ensure we want to support it long-term. An alpha stage for additions can do that.
Proposal
Alpha items would be hidden/inoperative entirely unless clients opted-in to receiving alpha (related: #383811). Opt-in might happen either by specifying specific sets of alpha changes, or to all alpha changes through a header.
Edited by Luke Duncalfe