Add Predicate trait to allow combination + fallbacks

[clux]

Converts predicates from a Fn(&K -> Option<u64>) to a trait Predicate that is implemented by fns of the same signature.

In controller-runtime terminology; this allows us to "or" predicates through Predicate::fallback, and allows us to "and" predicates through Predicate::combine.

Have decided to go for Fallback/Combine rather than Or/And because it feels confusing to use or on a fn returning Option (and i don't think it's the logical relation maps to the fallback/combination sufficiently well as touched on in #1225).

Usage with controllers:

use kube::runtime::{predicates, Predicate};

let predicate1 = predicates::labels.combine(predicates::annotations);
let predicate2 = predicates::generation.fallback(predicates::resource_version);

let stream = reflector(writer, watcher(api, cfg))
    .applied_objects()
    .predicate_filter(predicateX);

Controller::for_stream(stream, reader)
    .shutdown_on_signal()
    .run(reconcile, error_policy, context)
    .for_each(|_| futures::future::ready(()))
    .await;

This is a minor change to an unstable feature. The signature of predicates is preserved, but the stream interface now takes an impl Predicate rather than a function of type Fn(&K -> Option<u64>) - but these functions get a blanket impl of Predicate so this should not be noticeable for users.

[codecov]

Codecov Report

Merging #1228 (1aa0192) into main (2ad3827) will decrease coverage by 0.10%.
The diff coverage is 47.05%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1228      +/-   ##
==========================================
- Coverage   73.12%   73.03%   -0.10%     
==========================================
  Files          69       69              
  Lines        5515     5525      +10     
==========================================
+ Hits         4033     4035       +2     
- Misses       1482     1490       +8     

Impacted Files	Coverage Δ
kube-core/src/request.rs	98.71% <ø> (ø)
kube-runtime/src/utils/mod.rs	64.15% <ø> (ø)
kube-runtime/src/utils/watch_ext.rs	0.00% <0.00%> (ø)
kube-runtime/src/utils/predicate.rs	74.19% <50.00%> (-10.43%)	⬇️