Add selections subtype checking (Phase 2)

[abernix]

This follows up ae9c728 — which implemented Phase 1 — but is currently blocked awaiting the apollo-rs primitives.

Goal

Follow up with Phase 2 of the project, once we have apollo-rs.

Phase 1 (Already completed / BEFORE THIS ISSUE / Stopgap)

1. apollo-rs will provide an AST API
2. Parse schema using apollo-rs at existing schema load point, and pass into federated execution.
3. Selection construction will currently check __typename for an exact match against the query plan. This will be relaxed to also include subtypes.
4. We will navigate the AST manually to check subtypes.

Phase 2 (THIS ISSUE)

1. apollo-rs will eventually provide an higher-level intermediary representation and API. At minimum the following features are required:
   1. Get a GraphQL type by name
   2. isSubtypeOf method to check if a type is a subtype of another type.
2. Rework the router code to use the HIR.

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This is issue should track the implementation of Phase 2

[abernix]

@SimonSapin has confirmed that this work is done.