fix(appsync): addSubscription only allows for field type (aws#16097)

Error in the documentation and type checking. Fixed both the readme and the related PR (integ test used a `Field` type so it still works as intended).

Related PR: aws#10078

Fixes: aws#16071

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-appsync/README.md

@@ -960,7 +960,7 @@ To add fields for these subscriptions, we can simply run the `addSubscription` f
 to the schema's `Subscription` type.
 
 ```ts
-api.addSubscription('addedFilm', new appsync.ResolvableField({
+api.addSubscription('addedFilm', new appsync.Field({
   returnType: film.attribute(),
   args: { id: appsync.GraphqlType.id({ isRequired: true }) },
   directive: [appsync.Directive.subscribe('addFilm')],

packages/@aws-cdk/aws-appsync/lib/schema.ts

@@ -4,7 +4,7 @@ import { CfnGraphQLSchema } from './appsync.generated';
 import { GraphqlApi } from './graphqlapi';
 import { SchemaMode, shapeAddition } from './private';
 import { IIntermediateType } from './schema-base';
-import { ResolvableField } from './schema-field';
+import { Field, ResolvableField } from './schema-field';
 import { ObjectType } from './schema-intermediate';
 
 /**
@@ -164,7 +164,7 @@ export class Schema {
    * @param fieldName the name of the Subscription
    * @param field the resolvable field to for this Subscription
    */
-  public addSubscription(fieldName: string, field: ResolvableField): ObjectType {
+  public addSubscription(fieldName: string, field: Field): ObjectType {
     if (this.mode !== SchemaMode.CODE) {
       throw new Error(`Unable to add subscription. Schema definition mode must be ${SchemaMode.CODE}. Received: ${this.mode}`);
     }

packages/@aws-cdk/aws-appsync/test/appsync-code-first.test.ts

@@ -141,6 +141,31 @@ describe('code-first implementation through GraphQL Api functions`', () => {
       Definition: `${out}`,
     });
   });
+
+  test('addSubscription allows for adding fields but not resolvable fields', () => {
+    const ds = api.addNoneDataSource('DS');
+
+    // WHEN
+    api.addMutation('addId', new appsync.ResolvableField({
+      returnType: t.required_id,
+      args: { id: t.required_id },
+      dataSource: ds,
+    }));
+    api.addSubscription('addedId', new appsync.Field({
+      returnType: t.required_id,
+      args: { id: t.required_id },
+      directives: [appsync.Directive.subscribe('addId')],
+    }));
+
+    const schemaDef = 'schema {\n  mutation: Mutation\n  subscription: Subscription\n}\n';
+    const mutationDef = 'type Mutation {\n  addId(id: ID!): ID!\n}\n';
+    const subscriptionDef = 'type Subscription {\n  addedId(id: ID!): ID!\n  @aws_subscribe(mutations: ["addId"])\n}\n';
+
+    // THEN
+    Template.fromStack(stack).hasResourceProperties('AWS::AppSync::GraphQLSchema', {
+      Definition: `${schemaDef}${mutationDef}${subscriptionDef}`,
+    });
+  });
 });
 
 describe('code-first implementation through Schema functions`', () => {