Featuer: Support Multiple Schemas

## 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'
```

Start by tracing how Rover reads the `file:` schema descriptor used for composition, using the configuration example in the issue as the expected input. Done means a subgraph can specify multiple `.graphql` files in `schema.file` and composition accepts them together; no source file or test is named, so identify the relevant implementation and tests first.

Written by the indexing model from the issue text.