Expose API for external generators

[alrz]

Currently christianhelle/refitter writes interfaces to a file for refit generator to see.

I think given an API like this it can directly call refit generator and avoid writing to a file.

public static void GenerateRefit(
    this IncrementalGeneratorInitializationContext context,
    IncrementalValuesProvider<RefitInterfaceModel> interfaces);

RefitInterfaceModel would be a metadata model for every interface and endpoints generated in another generator e.g. refitter.