fix(apigateway): correct JsonSchema.additionalItems property type (#33879)

### Issue #33878

Closes #33878.

### Reason for this change
Fix a bug in the JsonSchema interface.

:rotating_light: _This is a breaking change, but should be acceptable, since the bug prevented use of the changed interface portion_ :rotating_light: 

### Description of changes

Changing the `JsonSchema.additionalItems` type from `JsonSchema[]` to `JsonSchema | boolean` to match [Json Schema Draft-04](https://json-schema.org/draft-04/schema). This enables deployment of API Gateway models that include the `additionalItems` property.

### Describe any new or updated permissions being added

None

### Description of how you validated changes

Added an integration test to validate that it fixes the deployment issue.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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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: jusdino

allowed-breaking-changes.txt

@@ -957,3 +957,7 @@ weakened:aws-cdk-lib.cx_api.MetadataEntryResult
 strengthened:aws-cdk-lib.aws_eks.ClusterProps
 strengthened:aws-cdk-lib.aws_eks.FargateClusterProps
 removed:aws-cdk-lib.lambda_layer_kubectl.KubectlLayer
+
+# Fixing the JsonSchema interface to be consistent with JSON Schema spec
+changed-type:aws-cdk-lib.aws_apigateway.JsonSchema.additionalItems
+strengthened:aws-cdk-lib.aws_apigateway.JsonSchema