Create JSON schema for policy input

[lcarva]

Let's introduce a json schema for the policy input. This schema should be used to validate the policy input generated by the ec validate image command.

Acceptance Criteria

• Policy input has a JSON schema.
• Generated policy input is validated against the JSON schema.

For now, the JSON schema can just reside within this repo. We don't need to publish it anywhere. We can do that later if there's a need.