Custom scalar integration policy / decide what (if anything) to do about smaller crate integrations

[LegNeato]

See https://github.com/graphql-rust/juniper/pull/165#issuecomment-416403522.

There will always be a push and pull of wanting to put integrations in juniper vs keeping them out.

I think this will come up more and more as Juniper gets popular, especially once we go full async and show some benchmarks compared to other languages 😜 . We should probably have some "official" policy of what gets added or not.

Thinking out loud, perhaps we should have an unstable-integrations flag that we can put less popular crates behind. We would have no contract for keeping them release-to-release and deleting them would not impact semver but people can opt into using them at their own risk. Users would be acutely aware that if we delete the integration they need to copy the code into their own project and/or create their own integration crate.