workload selector should support set-based requirements

(This is used to request new product features, please visit for questions on using Istio)

**Describe the feature request**

The type WorkloadSelector, used in many Istio type definitions, and defined here (https://github.com/istio/api/blob/1c84260bf409683cc7602dff2cd838d772f7f43b/type/v1beta1/selector.pb.go#L32) should align to the kubernetes LabelSelector[1], which supports set based requirements.

so it would be possible to not only write expressions like:

```
selector:
matchLabels:
serving.kubeflow.org/inferenceservice: test-service
```

but also expressions like:
```
selector:
matchExpressions:
- key: "serving.knative.dev/service"
operator: Exists
```
Note that this enhancement would 100% back compatible and would like the istio behavior with the more general kubernetes behavior.

[1]:https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/

**Describe alternatives you've considered**
There isn't really an alternative for the problem I'm trying to serve.

**Affected product area (please put an X in all that apply)**

This api type appears in many APIs, not sure which all are, but like I said the change would be backward compatible.

[ ] Configuration Infrastructure
[x] Docs
[ ] Installation
[ ] Networking
[ ] Performance and Scalability
[ ] Policies and Telemetry
[ ] Security
[ ] Test and Release
[ ] User Experience

**Additional context**