UnstableChangeCallbacks

Unstable: the set of group-change callbacks to register on a client.

Only appData exists today. This is one class rather than a bare callback so handlers for the other mutable fields (name, description, image url, admin lists, permissions, disappearing settings) can be added as further defaulted properties — additive for existing callers.

Registered at client creation via ClientOptions.unstableChangeCallbacks, because the changes it reports arrive from the stream and sync paths, where no SDK call is on the stack to carry them.

Pre-release: the shape of the payloads and of this class may change without a major version bump until it graduates.