New middleware to limit visibility of schema elements

The idea is to provide a middleware that allows to hide specific fields and types based on arbitrary logic. There hidden parts of the schema should be unavailable:

• During the execution
• During input value coercion (it should be possible to hide input as well as output types)
• During validation
• In the introspection results

The validation might be tricky since it is not aware of the middleware right now. On the other had, if for example a field is hidden, GraphQL query that uses the field should produce a violation during the validation phase. So maybe this feature needs to implemented as a new concept in the library and not necessarily as a middleware.

It should be possible to "hide" following elements of the schema:

• Type (input and output)
• Field (input and output)
• Argument
• Enum value

This idea is inspired by "Limiting Visibility" feature in graphql-ruby:

http://graphql-ruby.org/schema/limiting_visibility.html