Supergraph visibility controls

[gmac]

Adds visibility controls to a stitched schema, allowing parts of a supergraph to be hidden from select audiences without compromising stitching implementation details. This offers the capabilities of Apollo Federation's @inaccessible directive with more flexibility atop GraphQL Ruby's Visibility system.

type Product {
  id: ID! @visibility(profiles: []) # hidden from all requests
  msrp: Float! @visibility(profiles: ["private"]) # only visible to "private" requests
  price: Float! # visibile to all requests
}

This features wraps the GraphQL::Schema::Visibility system and uses nil profiles, so requires GraphQL Ruby v2.5.3 or later. While this is a pretty significant new feature, it adds no breaking behavior to existing stitching operations.

Resolves #122.
Resolves #133.

[rmosolgo]

Very cool -- I dreamed of a similar Ruby API (visible_in(...)?) when I added visibility profiles but ran out of steam during rmosolgo/graphql-ruby#5074 😩 Maybe I can take some inspiration from this implementation!

[gmac]

Ha, as the branch name implies, this was my third attempt (spanning two years) at incorporating visibility. The first two got bogged down in complexity and didn't feel very good. Once Visibility showed up though, there was a much clearer path. Once from_definition(base_types: ...) went in, I was out of excuses 😁