Add ability to define schema download output path that doesn't exist yet

[JakeDawkins]

resolves #1788

Previously, the CLI couldn't download schemas in apollo service:download or apollo client:download-schema when the path in the OUTPUT didn't exist.

This PR aims to add the ability to create a path given in the OUTPUT and to save the outputted schema to that path :)

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

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.