Add an experimental flag to codegen configuration and disallow @defer if not opted into

[calvincestari]

This makes it explicit to users that they are opting into an experimental feature that could change and introduce breaking changes into their project.

[calvincestari]

We've decided against an explicit flag to opt into that enables this feature. Defer will simply work but the documentation will explicitly note that @defer support is experimental and could change before it is merged into the GraphQL spec.

[github-actions]

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