rpcserver+macaroons: extract RawMacaroonFromContext

We'll re-use the code for extracting a macaroon from a request context
later on so we extract it into its own exported function.

rpcserver.go

@@ -6995,8 +6995,7 @@ func (r *rpcServer) CheckMacaroonPermissions(ctx context.Context,
 	}
 
 	err := r.macService.CheckMacAuth(
-		ctx, hex.EncodeToString(req.Macaroon), permissions,
-		req.FullMethod,
+		ctx, req.Macaroon, permissions, req.FullMethod,
 	)
 	if err != nil {
 		return nil, status.Error(codes.InvalidArgument, err.Error())