feature: Support retrieving schema as SDL, rather than via introspection

[domdomegg]

Some services provide the GraphQL schema as SDL directly (rather than as a response to an introspection query). It would be useful for this plugin to handle endpoints which expose this SDL. This is similar to how graphql-codegen has a handleAsSDL argument.

Apologies if this is a duplicate of an existing issue - had a look through other issues and the docs but couldn't find anything obvious.

[BoD]

Just a quick note to say that it's also possible to use introspection but save the result as an SDL doc (that's the approach that Apollo Kotlin followed).