Add a specific issue type for fragment cycles

[martinbonnin]

Add FragmentCycle. See #6756

It's not really possible to ignore it at the moment because the codegen relies on that fact in usedFragments and also when generating CompiledFields.

The following fails because there is a chicken/egg problem in the initialization of __root:

public object FooDetailsSelections {
  public val __root: List<CompiledSelection> = listOf(
        CompiledField.Builder(
          name = "__typename",
          type = GraphQLString.type.notNull()
        ).build(),
        CompiledFragment.Builder(
          typeCondition = "Foo",
          possibleTypes = listOf("Foo")
        ).selections(FooDetailsSelections.__root)
        .build()
      )
}

[apollo-librarian]

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