Retrieve events schema

Consumers will be able to retrieve the Async API for a specific namespace, event and event version using the exposed retrieve API.

Also consumers will be able to retrieve the schema of the content published as part of the data field of the payload for a specific event name and data version using the exposed retrieve API so that consumers will be able to verify the schema of the contents of the data field.

For retrieval of the schema of a multi-region event, the consumer can use region-specific registry endpoints.