Schema validation rules

[alexsteinerde]

This allows to specify custom validation rules that will be executed when a query is run. The logic already existed in GraphQL but wasn't exposed in Graphiti.

Use-Cases: E.g. disabling schema introspection in production (also done via validations in JS https://github.com/helfer/graphql-disable-introspection/blob/master/index.js). We already had a short discussion about this over at the GraphQL repo: https://github.com/GraphQLSwift/GraphQL/issues/122

Discussions:

• I experimented with multiple places where to expose the validation rules parameter. I added it to the normal schema execution function. But in the relatively new API world, I added it as an optional protocol requirement that can be implemented in the specify GraphQL API. What do you think about this approach?
• I used the types the GraphQL package uses for building custom validation rules instead of building new types only Graphiti uses to reduce overhead.

Feedback and change requests are welcome.

[alexsteinerde]

I changed the points as discussed. Feel free to comment on any other issues.