Add directiveResolvers param to mergeSchemas

[mfix22]

Hey guys,
This PR adds the ability to wrap a merged schema with directive resolvers. This is beneficial say, when you want to wrap your entire merged schema with a common @auth directive.

mergeSchemas({ schemas, resolvers, directiveResolvers })

However, this currently doesn't work with remote schemas (because directive information is lost when we printSchema graphql/graphql-js#869), although I am trying to get that working to. Thought I would get some feedback first before going through with the whole thing 😄

TODO:

• Update
• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• 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
• Update CHANGELOG.md with your change

[freiksenet]

The AST being lost should be resolved now. I'm wary of adding of new stuff to mergeSchemas atm cause we are heavily rethinking the api. I think atm you can just call attachDirectiveResolvers directly on resulting mergedSchema if you want to add them.

[mfix22]

@freiksenet that sounds like a good alternative. I wrote this before seeing the Next-API PR. I'll go ahead and close and I can revisit this after the new API lands if anyone wants it.