Register both as service and implementation

[notanaverageman]

When a service is registered as a base type, I want it to also be registered as self so that I can resolve it both via the base type and the self type.

This can be achieved with a RegisterAsSelf bool parameter in attributes. If you can direct me where to make the changes in generator code I can send a PR.

[Eyliess]

This would be a great quality of life improvement, I find myself running into this issue a lot.