Switch to @link in the supergraph

[pcmanus]

Generates supergraph with @link directives instead of @core ones.

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[pcmanus]

Unfortunately, this change is breaking for preview.

I initially though it wasn't because testing prior query planner version against the new supergraphs from this patch worked/works, but unfortunately calling the toAPISchema on the new supergraph from versions before this patch fails due to this part of the code unfortunately checking the 'feature' argument by name, argument that is renamed to 'url' for @link.

[benweatherman]

🐐 left an unimportant comment, do with it as you wish

[netlify]

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