[Feature] Create validation endpoint

[thetif]

Description and related issues

Create a validation endpoint that validates the APD using joi. This should reuse the schemas used in the frontend. The new validation should be stored in the APD and return the updated APD. The format should match the one expected by redux on the frontend.

This task is done when…

• [ ] the validation endpoint is created and runs the joi validations against the APD
• [ ] the updated validation errors get stored in the APD
• [ ] the validation errors are returned with the updated APD from the validation endpoint