Disable field merging for deferred fragment fields

[calvincestari]

This work is linked to #3141 but can be done independently and ultimately is required for that solution to work correctly. I recommend pairing with @AnthonyMDev and working towards solving for #2560 as well as much as possible.

4. Optional fragments (disabling field merging) - optional types are only needed when fragment fields are merged into entity selection sets. If field merging were disabled automatically for deferred fragments then the solution is simplified and we only need to alter the deferred fragments to be optional. Consuming the result data is intuitive too where a nil fragment value would indicate that the fragment data has not yet been received (i.e.: deferred) and when the complete response is received the fragment value is populated and the result sent to the client. This seems a more elegant and ergonomic way to indicate the status of deferred data but complicates the understanding of field merging.

[calvincestari]

I believe this is already done and available in the preview.1 release; there are no merged fields from deferred fragments.

[github-actions]

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