feat(apigateway): additionalProperties in RestApi Model supports JsonSchema type (#8848)

As per: https://json-schema.org/understanding-json-schema/reference/object.html, the additionalProperties can be boolean or JsonSchema.

JsonSchema was previously supported by APIGateway but It was removed for some reason here: 73a1de1.

fixes #8069

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

Author: Madhav Murthy

packages/@aws-cdk/aws-apigateway/lib/json-schema.ts

@@ -63,7 +63,7 @@ export interface JsonSchema {
   readonly minProperties?: number;
   readonly required?: string[];
   readonly properties?: { [name: string]: JsonSchema };
-  readonly additionalProperties?: boolean;
+  readonly additionalProperties?: JsonSchema | boolean;
   readonly patternProperties?: { [name: string]: JsonSchema };
   readonly dependencies?: { [name: string]: JsonSchema | string[] };
   readonly propertyNames?: JsonSchema;