graph: expose skip_duplicates on EntityType and InputSchema APIs

Add TypeInfo::skip_duplicates(), InputSchema::skip_duplicates(), and
EntityType::skip_duplicates() following the is_immutable() three-layer
delegation pattern. Object types return immutable && skip_duplicates;
Interface and Aggregation types return false.

Author: isum

graph/src/schema/entity_type.rs

@@ -68,6 +68,10 @@ impl EntityType {
         self.schema.is_immutable(self.atom)
     }
 
+    pub fn skip_duplicates(&self) -> bool {
+        self.schema.skip_duplicates(self.atom)
+    }
+
     pub fn id_type(&self) -> Result<IdType, Error> {
         self.schema.id_type(self.atom)
     }

graph/src/schema/input/mod.rs

@@ -132,6 +132,14 @@ impl TypeInfo {
         }
     }
 
+    fn skip_duplicates(&self) -> bool {
+        match self {
+            TypeInfo::Object(obj_type) => obj_type.immutable && obj_type.skip_duplicates,
+            TypeInfo::Interface(_) => false,
+            TypeInfo::Aggregation(_) => false,
+        }
+    }
+
     fn kind(&self) -> TypeKind {
         match self {
             TypeInfo::Object(_) => TypeKind::Object,
@@ -1212,6 +1220,13 @@ impl InputSchema {
             .unwrap_or(false)
     }
 
+    pub(in crate::schema) fn skip_duplicates(&self, entity_type: Atom) -> bool {
+        self.type_info(entity_type)
+            .ok()
+            .map(|ti| ti.skip_duplicates())
+            .unwrap_or(false)
+    }
+
     /// Return true if `type_name` is the name of an object or interface type
     pub fn is_reference(&self, type_name: &str) -> bool {
         self.inner