Sharing user activation with an iframe (or implement `allow-X-without-user-activation` for every `X`)

I have a use case where users can write plugins which have their code executed in a sandboxed iframe. I'd like the ability to allow these sandboxed iframes to play audio if the parent page has received user activation, since playing audio is a common need within plugins.

Something akin to allow-downloads-without-user-activation, but for playing audio would work (of course, the parent frame would still need to be activated for this to take effect), but it seems that there are several other use cases (e.g. here's a stack overflow question about opening file picker) which developers want, so if we're unlikely to get a flag like allow-X-without-user-activation for every value of X, then it seems like share-user-activation or something might make sense.

Start by reading the HTML Standard sections covering sandboxed iframes, user activation, and the allow-downloads-without-user-activation permission. Compare the proposed shared activation behavior with the file-picker use case linked in the issue. Done would require a settled specification design for sharing activation or a general per-feature permission mechanism.