dav: Convert is_collection const to function which will make filesystem access easier

crates/carddav/src/principal/mod.rs

@@ -29,7 +29,9 @@ impl Resource for PrincipalResource {
     type Error = Error;
     type Principal = User;
 
-    const IS_COLLECTION: bool = true;
+    fn is_collection(&self) -> bool {
+        true
+    }
 
     fn get_resourcetype(&self) -> Resourcetype {
         Resourcetype(&[