Refactor component signatures

[gmac]

⚠️ Minor breaking change

This gives a supergraph reference to Request, and then refactors Planner, Executor, and Shaper to simply wrap a request.

This shift makes requests act as the central hub of operations, similar to Query in GraphQL Ruby. This sets requests up to handle more contextual concerns such as visibility and authorization.

Migration steps

1. See doc changes in PR.
2. Request now takes a supergraph instance as its first required argument.
3. Planner, Executor, and Shaper now simply take a request argument. Better yet, don't use these components directly and invoke their behaviors through the request.