Create mechanism to enable / disable APIs #2279
Description
Proposed change: Add a mechanism for us to mark an API as unstable, and to ensure that users explicit enable them.
Motivation: We want to enable experiments and fast development for our libraries the same way we do for our software without affecting users trust. An experimental API may bring instability and unexpected results, so users should be aware of that when they choose to enable them.
The initial use case would be async APIs introduced in #1659.