Support upload of JSON schema as data model in Designer

[dskogan]

Description

We should support uploading ready made JSON schema files as data model. Currently only XSD can be uploaded in Designer which triggers generation of JSON schema.

Acceptance criteria

• What is allowed/not allowed
• Validations
• Error messages and warnings
• ...

Tasks

• [ ] Allow users to add json schema as an alternative to xsd
• [ ] Hook up generation of json schema from xsd