[Fusion] Add support for result from different subgraph on interfaces

[Enterprize1]

Closes #7365.

When requesting a result from a different subgraph on an interface with multiple implementing types the request to the second subgraph was planned multiple times (once for each implementing type). This lead the compose-step to return null for the data from the second subgraph as the data was requested and added multiple times for each object returned by the interface list.

(Hope that explains the problem, if not maybe comparison of the query plan for the test I added before and after the change in ExecutionStepDiscoveryMiddleware.cs could provide some clarification :) )

Now only a single request to the second subgraph is planned for all types implementing the interface by deduplicating the contents of the backlog during execution step discovery.

[Enterprize1]

The failed checks seem unrelated to me, the Fusion.CommandLine test failed before on the main-version-15 branch.

[tobias-tengler]

Thank you for the contribution 😊
Funnily enough we had the same problem at my company and I started to write some tests for the issue in #8272.
With your fix applied all the problematic tests there seem to pass 🎉

[PascalSenn]

Thanks you for the contribution, @Enterprize1! 🚀

Preview version 15.1.6-p.1 should be available on NuGet in about 15–20 minutes.

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