Extend the readiness evaluation of dependencies with CEL expressions

[stefanprodan]

This PR extends the Kustomization API with the .spec.dependsOn[].readyExpr optional field that can be used to define a CEL expression to determine the readiness of a Kustomization dependency.

This is helpful for when custom logic is needed to determine if a dependency is ready. For example, when performing a lockstep upgrade, the readyExpr can be used to verify that a dependency has a matching version label before proceeding with the reconciliation of the dependent Kustomization.

apiVersion: kustomize.toolkit.fluxcd.io/v1
kind: Kustomization
metadata:
  name: app-backend
  namespace: apps
  labels:
    app/version: v1.2.3
---
apiVersion: kustomize.toolkit.fluxcd.io/v1
kind: Kustomization
metadata:
  name: app-frontend
  namespace: apps
  labels:
    app/version: v1.2.3
spec:
  dependsOn:
    - name: app-backend
      readyExpr: >
        dep.metadata.labels['app/version'] == self.metadata.labels['app/version'] &&
        dep.status.conditions.filter(e, e.type == 'Ready').all(e, e.status == 'True') &&
        dep.metadata.generation == dep.status.observedGeneration

The CEL expression contains the following variables:

• dep: The dependency Kustomization object being evaluated.
• self: The Kustomization object being reconciled.

Note: When readyExpr is specified, the built-in readiness check is replaced by the logic defined in the CEL expression. You can configure the controller to run both the CEL expression evaluation and the built-in readiness check, with the AdditiveCELDependencyCheck feature gate.

Closes: #1490

[matheuscscp]

nit: The word additive here all by itself can sound a bit vague. Since here is a space where we don't have to spare words (like in the case of choosing a name for a feature gate), we can probably be a bit more verbose/redundant here and say that "the hard-coded logic and the CEL expression are AND'ed together".

[stefanprodan]

the hard-coded logic and the CEL expression are AND'ed together

This sounds wrong to me, it's not hard-coded but bulit-in, the AND'ed is no better than additive.

[matheuscscp]

Sure, hard-coded vs built-in sounds the same to me, but I'd say AND'ed is pretty clear and additive not so much

[matheuscscp]

LGTM! 🚀