Add config for remote schemas

**Is your feature request related to a problem? Please describe.**
I don't want to have to manually update the schema.graphql file.

**Describe the solution you'd like**
Add `RemoteSchemaURL` to `generate/config.go`. Temporarily download this file to disk for generation.

**Describe alternatives you've considered**
I currently have a ci job to download the remote schema and compare it to the schema in my project

Start in generate/config.go to trace how configuration selects the schema.graphql input and how generation is invoked. Follow the generation flow to determine where a RemoteSchemaURL download can be used temporarily, and verify that generated output comes from the remote schema without requiring a manually updated local file.

Written by the indexing model from the issue text.