Add Query Plan tests from integration test suite

[JakeDawkins]

Right now, the integration test suite for apollo-gateway contains a mix of query planning tests and execution tests. This PR aims to extract just the query planning portions into cucumber's .feature files to be used when implementing a query planner in other languages.

This builds off the work from #4297 and #4448

I also took some comments I wrote about the internals of the query planner and added them in this PR (apparently I left them off my planner/executor PR)

[abernix]

Hi @JakeDawkins!,

This is a semi-automated message. Apologies for the lack of personal detail. I’d encourage reading the rest of this message but, if you're busy, hopefully this is as easy for you — or someone who is willing to own the contribution — as pushing the button later in this message to re-create this PR, from an auto-migrated branch, on the new Federation repository. If you have time, read on!

We’re in the process of moving federation-related concerns out of this repository (where it has historically lived) into a new home specifically for Apollo Federation. This PR is affected by the transition since it touches code which has been moved.

I’ve done some preparations to make this as easy as possible, but we’ll need to close this PR, and I could use your help in re-opening it on the new Apollo Federation repository.

In apollographql/federation#134 (which has a lot more details, if you’re interested), we introduced the same code that was in this repository to the new Federation repository (maintaining its history) and removed the code from this repository in #4561.

While this PR still needs to get reviewed and approved to land, it should no longer live in this repository in its current state since it cannot merge in anymore. To facilitate the movement of this PR to the new home, I’ve automatically generated branches on the new repository using the same commit authorship and messages that you originally included on this PR.

Pull-requests can’t be moved on GitHub in the same way Issues can be. While I could just create the PRs from these new branches too, the contribution belongs to you!

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Original PR author only: Click this button to open a new PR from the auto-created apollo-server-import/pr-4469 branch on the new Apollo Federation repository

(The code and your commits should be the same and you will have an opportunity to confirm, but this way you can continue to be the author and track its progress on the new Federation repository!)

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There’s no easy way to bring over any existing comments on this PR, so I encourage you to copy and paste those onto the new issue if possible.

Overall, I hope this process is relatively easy for you while maintaining your commit contribution authorship. I apologize for any inconvenience caused by this shuffle, but please ping me if I can help in any way!

🚀