Signature for callbacks that report that a value has been set.
This is the same signature as
ValueChanged, but is used when the
callback is called even if the underlying value has not changed.
For example, service extensions use this callback because they
call the callback whenever the extension is called with a
value, regardless of whether the given value is new or not.
ValueGetter, the getter equivalent of this signature.
AsyncValueSetter, an asynchronous version of this signature.