Allow an optional function to resolve the rootValue

[tgriesser]

This allows providing a function to the rootValue option, which is called with the parsed query AST DocumentNode and the return value determines the value of the root.

This is useful in situations where you want to provide a different rootValue depending on the operation type, for instance if the rootValue should be different for a query vs a mutation

It also includes the commit from the type fixes in #1554

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[evans]

@tgriesser Thanks for adding this support! Let's get some documentation for it in the API reference and then we can merge it

[tgriesser]

Great! Rebased & added API reference docs - let me know if you want me to add anything else.

[tgriesser]

Not sure why the netlify check didn't pass

[tgriesser]

@evans bump

[evans]

@tgriesser Thank you!

[alexisbertholom]

It would be nice if we could pass the QueryOptions to this function, so we can, for example, provide a different rootValue depending on the Context. I'd gladly send a PR, I already have the code ready.