Add `implements` support for models

[povilasjurcys]

This PR adds basic interface support for GraphqlRails::Model.

Usage:

module VisitorInterface
  include GraphQL::Schema::Interface
  
  field :failed_logins_count, Integer
end

class UserDecorator
  include GraphqlRails::Model

  graphql do |c|
    c.implements(VisitorInterface)
    # ...
  end
end