@Target(value=METHOD)
@Retention(value=RUNTIME)
public @interface DirectoryGateway
Supports CardDAV Directory Gateway Extension
https://github.com/miltonio/milton2/issues/25
https://tools.ietf.org/html/draft-daboo-carddav-directory-gateway-02#page-4
Implement this for each address book class to return whether it is or is not
to be used as a directory gateway
MUST return Boolean !!
Default bean property is "directoryGateway"
Example annotated method: