Add support for passing parsed schema ast to mergeSchemas

[ganemone]

Many APIs across graphql-tools and apollo libraries support passing
a parsed graphql ast or a graphql string. For example, makeExecutableSchema.
This PR updates the mergeSchemas API to support passing a parsed graphql ast.

[apollo-cla]

@ganemone: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[KevinGrandon]

Leaving an approval here in case we need to fork this, but hoping we can upstream. Nice!

[hwillson]

Thanks very much @ganemone!