Generate GenerateAutomaticInterfaceAttribute

[crwsolutions]

Generate GenerateAutomaticInterfaceAttribute instead of asking that of the user.

See cookbook: https://github.com/dotnet/roslyn/blob/main/docs/features/incremental-generators.cookbook.md#generated-class

If you're feeling adventurous, it is possible to remove the namespace from the generated attribute (so remove 'namespace AutomaticInterface'). That would give even more convenience. Given the name of the attribute I think there is very little change of type conflicts.