A return value handler that supports async types. Such return value types
need to be handled with priority so the async value can be "unwrapped".
Note: implementing this contract is not required but it
should be implemented when the handler needs to be prioritized ahead of others.
For example custom (async) handlers, by default ordered after built-in
handlers, should take precedence over
@ModelAttribute handling, which should occur once the async value is
ready. By contrast, built-in (async) handlers are already ordered ahead of