Implement #[derive(GraphQLInterface)] to use structs as GraphQL interfaces (#1026)

- support `#[graphql_interface]` on structs

Author: ilslv

integration_tests/codegen_fail/fail/input-object/derive_incompatible_object.stderr

@@ -1,10 +1,8 @@
 error[E0277]: the trait bound `ObjectA: IsInputType<__S>` is not satisfied
- --> fail/input-object/derive_incompatible_object.rs:6:10
-  |
-6 | #[derive(juniper::GraphQLInputObject)]
-  |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `IsInputType<__S>` is not implemented for `ObjectA`
+ --> fail/input-object/derive_incompatible_object.rs:8:12
   |
-  = note: this error originates in the derive macro `juniper::GraphQLInputObject` (in Nightly builds, run with -Z macro-backtrace for more info)
+8 |     field: ObjectA,
+  |            ^^^^^^^ the trait `IsInputType<__S>` is not implemented for `ObjectA`
 
 error[E0277]: the trait bound `ObjectA: FromInputValue<__S>` is not satisfied
     --> fail/input-object/derive_incompatible_object.rs:6:10

integration_tests/codegen_fail/fail/interface/additional_non_nullable_argument.stderr

@@ -0,0 +1,5 @@
+error: GraphQL interface #[graphql_interface] attribute is applicable to trait and struct definitions only
+ --> fail/interface/attr_wrong_item.rs:9:1
+  |
+9 | enum Character {}
+  | ^^^^^^^^^^^^^^^^^

integration_tests/codegen_fail/fail/interface/wrong_item_enum.rs renamed to integration_tests/codegen_fail/fail/interface/attr_wrong_item.rs

@@ -0,0 +1,12 @@
+use juniper::{GraphQLInterface, GraphQLObject};
+
+#[derive(GraphQLObject)]
+pub struct ObjA {
+    test: String,
+}
+
+#[derive(GraphQLInterface)]
+#[graphql(for = ObjA)]
+enum Character {}
+
+fn main() {}

integration_tests/codegen_fail/fail/interface/attr_wrong_item.stderr

@@ -0,0 +1,6 @@
+error: GraphQL interface can only be derived on structs
+  --> fail/interface/derive_wrong_item.rs:9:1
+   |
+9  | / #[graphql(for = ObjA)]
+10 | | enum Character {}
+   | |_________________^