JSON Schema validation for the config file

It would be great if we had a full schema for the config file. This would:
- serve as documentation for end users,
- add a check before runs, so potential errors are caught sooner, rather than later (e.g typos, incompatible configs between major versions (also should help with upgrading between them), etc)
- serve as a reference for testing and developing.

See (general info):
- https://json-schema.org/

See (potential packages):
- https://github.com/epoberezkin/ajv
- https://github.com/korzio/djv