inline_completion_button: Put "Eager Preview Mode" menu entry behind a feature flag (#24734)

This PR puts the "Eager Preview Mode" menu entry behind a feature flag
rather than a staff flag.

Currently it defaults to `false` for staff so that it doesn't leak into
any marketing/launch materials.

Folks who want to see it can opt-in to the flag explicitly, for now.

Release Notes:

- N/A