Provide option for `casting_syntax=as` for `relay.config.js` when `language` is `flow`

We’d like to enable `casting_syntax=as` in our project’s `.flowconfig`, however the generated Flow types from the Relay compiler use the old `:` syntax, resulting in thousands of `invalid-type-cast-syntax` errors, which makes it difficult to identify the cases in our codebase that require updating.

It would be nice if there were an option for `relay.config.js` where we could opt-in to the new syntax, as it will eventually [become the default](https://medium.com/flow-type/new-type-casting-syntax-for-flow-as-3ef41567ff3e).

> At some point in the future we will remove the option and only support “as” casts, so code update your codebase!

Thanks for looking!