Feature Request: Support multiple schema files

[StevenACoffman]

While consistency with gqlgen is really a goal as such, but it would be convenient if we similarly supported multiple schema files they way it does:

package: genqlient
schema:
- ../../pkg/graphql/shared-schemas/side_by_side_directives.graphql
- ../../pkg/graphql/shared-schemas/user_deletion.graphql
- ../../pkg/graphql/shared-schemas/automap.graphql
- schema.graphql
- homepage_render.graphql
# The files from which we pull operations (relative to genqlient.yaml).
operations:
- genqlient.graphql

A client that is generated from the federated composed_schema.graphql has a lot more churn than what is required to communicate with a single service.
It might be nice to generate a "just this service" client, but that would require supporting multiple schema files.