Featuer: Support Multiple Schemas

[shellscape]

Description

I'd like to request support for multiple schema files in the file: descriptor. Being able to specify multiple schema files for composition is something nearly every tool for GraphQL supports that I've run across in the last few years; including graphql codegen.

Specifically supporting:

subgraphs:
  account:
    routing_url: http://localhost:5011/batman/graphql
    schema:
      file:
        - './primary.graphql'
        - './secondary.graphql'