Allow for annotation to replace empty no-args constructor with Inject

There has been a lot of discussion about the no-args @Inject constructors already. To that end, I would like to propose an alternative solution to the boilerplate-y looking constructor - an annotation that would explicitly mark the class as injectable by Dagger, even without @Inject marked constructor. Both options could live on, for those who prefer one or another.

Something like this:

import dagger.Injectable;

@Injectable
public class MyService {
    // ...
}

If the annotation is present, Dagger would allow for “default” no-args constructor to be used.

Related to #1105, #1409 and discussed in square#412.

[ronshapiro]

I don’t think adding support for something that is so similar to an already standard solution makes sense. Then readers have to discern why one solution was used and not the other. As it is, @Inject MyService() {} is just as easy to write, plus it will work with all other JSR 330 injectors.

[ronshapiro]

Each new feature adds maintenance cost and conceptual overhead. Some consider it flexibility, others consider it confusing. We’ve opted for fewer concepts unless there’s a clear benefit (runtime, compile time static analysis options, etc). I don’t think this has much benefits besides some personal style.

If we allow custom binding annotations at some point, this would be a great candidate for something that you could add to your project (but keep outside of Dagger).