[feature request] Use local file as schema source

[erheron]

Hello 👋🏻

I use GraphiQL v0.7.2 on Mac with M1 processor I haven't found the option to use a local file URL (e.g. file://<host>/<path>) as a source for schema 👀

This feature, however, would be highly valuable in certain scenarios, e.g.:

• somebody does not have internet connection (e.g. works offline or from a train)
• somebody's schema is protected by some authn headers which take a minute or two to update

Could we add this to graphiql-app? 🙏🏻