kwctl: support running policy groups

[flavio]

Instead of creating a separate command for policy groups, we could add a new flag to the run command that takes a CRD as an input. e.g. kwctl run --crd policy_group.yaml or kwct run --crd policy.yaml
The supported CRD are all the policies CRD, including policy groups.

Prerequisite: We need to move the expression evaluation logic from the policy server crate to the policy evaluator crate.

This is a follow up of #906