Signature for service extensions.
The returned map must not contain the keys "type" or "method", as
they will be replaced before the value is sent to the client. The
"type" key will be set to the string
_extensionType to indicate
that this is a return value from a service extension, and the
"method" key will be set to the full name of the method.
typedef ServiceExtensionCallback = Future<Map<String, dynamic>> Function(Map<String, String> parameters);