2.x: Matching DisposableObserver class for Single

[vanniktech]

How do you feel about adding a matching class for DisposableObserver with Single support?

Instead of Observer it'll implement SingleObserver

[akarnokd]

Sounds good; please add one for Completable and Maybe while you are at it.

[vanniktech]

Will do 👍

[vanniktech]

Closing via #4504