Find a home for Cradle, the query-planner compatibility acceptance tests

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apollographql/apollo-server#4297 introduced what are meant to be language-agnostic compatibility acceptance tests for query-plans. For now, Cucumber ".feature files" are the extent of this testing, but we believe this is just the beginning of a larger federation test-suite — which we're calling Cradle — that can satisfy testing requirements of various initiatives.

For example, when #103 has brought these tests into this repository, the work in #101 can take place and build a query-planner (in Rust!) that meets the expectations of the test-suite.

Thankfully, manually copying the files into this repository to maintain test-completeness is not our end-game. Furthermore, the Apollo Server repository should not remain the source of truth for those files. Cradle needs a home repository which will house its documentation and allow it to be utilized by other projects.

We need to figure out the details of how that work! (And this is the issue to discuss that in, if anyone has ideas!)

Incredibly naive, half-baked, straw-dog proposal: A separate repository that can be used as a Git submodule (or git subtree).

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This repository will be the home! See #145 for tracking that progress.