Validate JSON-AD (endpoint + CLI option)

[joepio]

I used to have an endpoint for this (when the front-end was generated server-side, and type checking sucked) #28 , but now I don't. Time to change this. See docs issue: https://github.com/ontola/atomic-data-docs/issues/110

How should validate-json-ad work? It takes one argument, a json-ad string, and returns an error if encountered. Parses it, checks datatypes, returns error. Should it also check every mentioned URL, every property, to see if they are still online? Seems a bit much. Maybe if I add it as an option, but that would not be necessary for the first version. I think the existing parse-json-ad stuff already does everything... So I guess all that I need to do is add the Endpoint and CLI option. Easy!

• [x] Add the validate functionality to atomic_lib. (not needed, we have parse!)
• [ ] Add an Endpoint to atomic_server, that accepts a JSON-AD string (or URL) and returns a message.
• [ ] Add a CLI command for validating JSON input (I think with a pipe command, not sure how to do this, but it's probably easy with clap, so I can do grep myfile.json-ad | atomic-cli validate).

[joepio]

I think #390 may be the right place to fix this. At least, most of the logic should be shared.