Add macros for custom client/server interfaces

[sfackler]

Some services need to interact with or expose APIs that cannot be defined in Conjure. Currently this requires a bunch of manual glue that we should be able to write for the service with a macro analagous to dialogue-annotations-processor:

#[service]
trait MyCustomService {
    #[endpoint(method = GET, path = "/foo/{bar}")]
    fn foo(&self, bar: i32) -> Result<(), Error>;
}