Reject directives within fields arg of @key, @provides and @requires

[pcmanus]

Noticed that we accepting (parsing) directive applications with a @key, @provide or @requires, but were silently ignoring it. In other words:

type T @key(fields: "x ... @skip(if: true) { y }") {
  x: Int
  y: Int
}

was accepted, but the @skip was completely ignored. Of course, it's unclear why would anyone write this, but it still imo qualify as a bug that we silently ignore this since the code essentially do not do what is written. It's certainly an oversight.

More generally, as we don't anything with any such directive applications, and for user/custom directives, there is no way to provide any processing whatsoever, so overall, I think we should just reject any directive applications to avoid any risk of confusion, and this is what the patch does.

I'll further note that:

1. the example above with @skip is a bit arbitrary when you need a hard-coded true or false for the if: why would you ever write this? But the context here is actually Deliver: @defer support in the query planner #1921: someone could want to try to see what a @defer does within a fields value, and we should be clear it just doesn't work.
2. I'm not trying to say that it will never make sense to allow directives in such values. There absolutely could be future use case for it, but we can just relax this validation when that happens. The point is that we shouldn't accept something we don't actually support.
3. one could make the argument that, insofar as this reject things that were not rejected before, this could be considered as a backward incompatible change. My argument here is that the existing behaviour is imo a bug and so I have no qualm putting this in a minor release (I mean, fixing bugs always modify behaviour after all). But of course, we'll still put a word about it in the changelog.

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[benweatherman]

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