Add basic validation implementation

[jonnydgreen]

Closes: https://github.com/mercurius-js/validation/issues/1

Draft PR that adds a basic implementation. At the moment, I have just focussed on proving the concept for external policies:

• JSON Schema (ajv)
• JTD (ajv)
• Functional approach (field arguments only)

This has the following caveats (which I think are okay for v1?):

• Functional approach only works for arguments. I think extending this beyond arguments is pretty complicated and maybe better for future versions?
• I am only considering input data. Again, for v1, I think validating on outputs is too much for the time being and just focussing on inputs is probably a better initial approach?

Let me know what you think about the above caveats and what I've done so far. The quality of the code isn't great atm as there are a lot of things to polish up, but the core ideas are there if you'd like to take an early look :)

Still to do:

• [x] Directive support (using JSON Schema and external policy logic)
• [x] List support
• [x] Schema type inference
• [x] Better options validation
• [x] Make sure it works in the gateway
• [x] Deduplication of code and general clean up
• [x] Get coverage to 100%
• [x] Add ability to register custom formats using the Ajv API
• [x] Typings
• [x] Examples
• [x] Benchmarks
• [x] Documentation (inc. function docstrings)

Open Questions:

• Do we want to support validation of full types for v1?

[jonnydgreen]

Made some more progress - next steps are to tidy things up a bit, add extra testing and then move onto:

• Documentation
• Types
• Examples

Is there any functionality not covered by the tests that you'd like in v1?

[jonnydgreen]

Almost there with the documentation. I'll push that up with the examples tonight and then crack on with the typings :)

[mcollina]

Amazing work!

[jonnydgreen]

Thanks! It's been very fun to code :)

[jonnydgreen]

Finalising the rest of the docs after a couple of changes - once that's done this afternoon I'll bring it out of draft :)