fix(apigateway): id in schema model maps to $id (#15113)

If we specify the `id` field when defining an Api Gateway Model's schema, it gets mapped to a `$id` key, which creates an invalid model because it doesn't comply with the DRAFT-04 specification. The specification requires this field to remain named as `id`.

fixes #14585


----

*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-apigateway/lib/util.ts

@@ -101,7 +101,6 @@ export class JsonSchemaMapper {
   private static readonly SchemaPropsWithPrefix: { [key: string]: string } = {
     schema: '$schema',
     ref: '$ref',
-    id: '$id',
   };
   // The value indicates whether direct children should be key-mapped.
   private static readonly SchemaPropsWithUserDefinedChildren: { [key: string]: boolean } = {

packages/@aws-cdk/aws-apigateway/test/util.test.ts

@@ -1,4 +1,4 @@
-import { JsonSchema, JsonSchemaType } from '../lib';
+import { JsonSchema, JsonSchemaType, JsonSchemaVersion } from '../lib';
 import { JsonSchemaMapper, parseAwsApiCall, parseMethodOptionsPath } from '../lib/util';
 
 describe('util', () => {
@@ -136,5 +136,18 @@ describe('util', () => {
         default: 'blue',
       });
     });
+
+    test('"id" maps to "id" when using DRAFT-04', () => {
+      const schema: JsonSchema = {
+        schema: JsonSchemaVersion.DRAFT4,
+        id: 'http://json-schema.org/draft-04/schema#',
+      };
+
+      const actual = JsonSchemaMapper.toCfnJsonSchema(schema);
+      expect(actual).toEqual({
+        $schema: 'http://json-schema.org/draft-04/schema#',
+        id: 'http://json-schema.org/draft-04/schema#',
+      });
+    });
   });
 });