graphql: embed *Resolver instead of backend interface (#25468)

This creates some infrastructure to share resources between graphql
API objects.

graphql/service.go

@@ -56,10 +56,6 @@ func (h handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 // New constructs a new GraphQL service instance.
 func New(stack *node.Node, backend ethapi.Backend, cors, vhosts []string) error {
-	if backend == nil {
-		panic("missing backend")
-	}
-	// check if http server with given endpoint exists and enable graphQL on it
 	return newHandler(stack, backend, cors, vhosts)
 }