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.

[nightkr]

users now need to import ListParams when using reflector/watcher even if they're using defaults

They can also call it as Default::default().

• reflector exposes the Writer even if it's unsafe to have multiple writers

The reflector takes the Writer by-move, so you can't pass the same Writer to multiple reflectors.

[clux]

Ahh. Ok. Thanks. Probably going to ignore those parts then.

[clux]

Wrote some docs for the Controller, and that's probably the hardest one. Needed a few more dev-dependencies we already had in Cargo.lock.

https://docs.rs/kube-runtime/0.37.0/kube_runtime/controller/struct.Controller.html

[clux]

Most of the issues here are now in separate issues. So the only question left here is whether or not to add builders to reflector, watcher. This forces one question:

• if Watcher<K> is just a builder to call watcher
• and Reflector<K> is just a builder to call reflector (hiding the watcher arg + import)
• then Controller<K> should really just call out to controller (i.e. keep the original name of controller rather than applier)