Support $ref to other files and repos

[taylor-sutton]

It would be helpful to reduce duplication across projects if we could refer to models/definitions from outside the current file.

[nathanleiby]

This blog post provides a nice overview and justification: https://davidgarcia.dev/posts/how-to-split-open-api-spec-into-multiple-files/

It also suggests a workaround (basically: write multiple files then use pre-processing to join them), but ideally Wag could support this directly.