feat(router): Add RoutesBuilder constructor

Author: tottoto

tonic/src/service/router.rs

@@ -27,6 +27,11 @@ pub struct RoutesBuilder {
 }
 
 impl RoutesBuilder {
+    /// Create a new empty builder.
+    pub fn empty() -> Self {
+        Self::default()
+    }
+
     /// Add a new service.
     pub fn add_service<S>(&mut self, svc: S) -> &mut Self
     where
@@ -64,6 +69,11 @@ impl Routes {
         Self { router }.add_service(svc)
     }
 
+    /// Create a new empty builder.
+    pub fn builder() -> RoutesBuilder {
+        RoutesBuilder::empty()
+    }
+
     /// Add a new service.
     pub fn add_service<S>(mut self, svc: S) -> Self
     where