api/resolver: update resolver to enable subdomain routing (#1747)

* api/resolver: update domain / service prefix usage

* api/resolver/subdomain: implement subdomain resolver for domain resolution

* api/handler: fix tests

api/resolver/host/host.go

@@ -17,6 +17,7 @@ func (r *Resolver) Resolve(req *http.Request) (*resolver.Endpoint, error) {
 		Host:   req.Host,
 		Method: req.Method,
 		Path:   req.URL.Path,
+		Domain: r.opts.Domain,
 	}, nil
 }