JSON Schema validation for the config file

[mhavelant]

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