Use JsonShapeValidatorMiddleware when validating an API response

[jkva]

Currently, API requests are assumed to be valid when returned from the API. Since the JsonShapeValidatorMiddleware is already used to validate webhook update shapes, this should be used as a default middleware within the JsonMapper instance used to parse API request responses.

Related to #3.

[jkva]

There is current code available for that, yet when enabled this triggers a fair amount of validation errors in existing tests which need to be addressed.