Rails-like argument validation

[rmosolgo]

Currently, the only solution here is to implement these in application logic. That works fine, but we could provide a framework to help with it.

Fixes #799
Fixes #3164

To figure out in this PR:

• Add validators like Rails built-in validators
• Add general options (allow_blank, etc)
• Get the multiple-errors situation figured out
  • I don't this this is great -- redundantly validating list items -- but it works, and we can fine-tune it later.
• Figure out multi-argument errors (for example, one arg must be greater than the other arg, or, one of these three args must be present)
  • For now, this can be done with validates(...) on a field, using a custom validator. It could be better, like Rails, but I think it's good enough for now.
• Make sure arguments, fields, input objects, and resolvers all have a good validation story
  • Fields accept configs via validates: or validates ...
  • Validator inheritance
  • Should args with default_value: "count" in required: { one_of: ... } combinations?
• Should the default be to include .graphql_name or .path in error messages?
• Add docs for built-in validators
• Document & test the custom validator path
• Should this work include "static" validations? (This implementation takes effect at runtime, which is the easiest and most flexible. But some validations could be run ahead-of-time.) no, we can refine this later.