builders for kube-runtime reflector

[clux]

Trying to propagate kube-runtime to all external repo actix examples and noticing a few things that should be made better for a smoother experience:

• users need to import watcher when using reflector (even if they expose the same events and they only use one)
• users need to import Writer when using reflector (even though they cannot really use it sensibly)

Proposed solutions:

• add builder Cache<T> around reflector - Cache: implement a builder wrapper for reflector #698
• Cache builder to allow retrieving a reader as in Controller - Cache: implement a builder wrapper for reflector #698
• add Watcher<T> builder - Add backoff handling for watcher and Controller #703

Edit: crossing out a few from feedback.
Edit2: removing ancient comments here, as a long time has passed here.

[clux]

Managed to get around this with an easier solution in #907 thanks to Teo's ideas on WatchStreamExt.