Implement JSON schema validation

[teemukataja]

Proposed solution

Implement JSON schema validation to endpoints for automatic structure checking and user feedback.

DoD (Definition of Done)

• To elixir_rems_proxy/schemas/ write JSON schemas;
• To elixir_rems_proxy/utils/validate.py implement validation functions;
• Validate POST body;
• Validate PATCH body (currently identical to post, should change in the future with new version of API specification);
• Validate GET response.

Testing

Write unit tests for elixir_rems_proxy/utils/validate.py and update tests for endpoints.