[Fusion vNext] Add support for @skip / @include

[tobias-tengler]

If root selections on an operation are conditional, they will be translated into a new Condition node that will make it very easy for the execution to skip over entire operation trees, if a condition is not met. Same goes for fields that are selected from a different subgraph, if they're conditional the entire lookup operation will be placed below a Condition node.
Conditional sub-selections will be forwarded using the appropriate directive.
Static conditions like name @skip(if: true) will cause the field to be entirely removed from the selection, name @skip(if: false) will cause just the "dead" directive to be removed.

This doesn't include support for (inline) fragments yet.

[codecov]

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  Hits            ?   111432           
  Misses          ?    32668           
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