feat: [cel] add CEL-based assertions and offline VAP evaluation

[crandles]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Adds a cel package for writing CEL-based assertions against Kubernetes objects in tests. Same expression language the API server uses for CRD validation and admission policies, so tests don't drift from the policies they exercise.

Split out from #562 -- this PR includes the following packages: cel (Evaluator, Bindings, Libraries, program cache), cel/policy (offline VAP evaluation, FromVAP), and a small offline examples/cel test that exercises both.

The cel/decoder, cel/feature, cel/wait sub-packages and the live-cluster example will follow in a second PR layered on this one. (WIP: crandles#3)

Design doc: docs/cel-assertions.md.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Added a `cel` package for writing CEL-based assertions against Kubernetes objects in tests, including offline ValidatingAdmissionPolicy evaluation.

[k8s-ci-robot]

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[k8s-ci-robot]

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[vladimirvivien]

/ok-to-test

[vladimirvivien]

@crandles - At the risk of completely misrepresenting your intent with your API design, I am going to propose a design that simplifies the API a bit. You welcome to push back if you think I am missing your point.

1. One entry point Evaluator
2. Add your objects and expressions ahead of time
3. Collapse Eval, Check
4. Change Check to always return Result (covering Policy.Check)
5. Keep Check() with no param; or change with Check(expressions...string)

Single assertion:

Current:

  ev, err := celpkg.NewEvaluator()

  if err := ev.Assert(
      "object.spec.replicas >= 1",
      celpkg.ObjectBinding(dep),
  ); err != nil {
      t.Fatal(err)
  }

Proposed:

ev, err := cel.NewEvaluator(cel.WithObjectBinding(dep))

// or ev.AddObjectBinding(obj)

ev.AddExpression("object.spec.replicas >= 1")
result, err := ev.Check(). // or ev.Evaluate()
if result.Passed() { ... }

Multi-validation policy:

Today:

  pol := policy.Policy{
      Name: "deployment-replicas",
      Validations: []policy.Validation{
          {Expression: "object.spec.replicas >= 1",   Message: "replicas must be at least 1"},
          {Expression: "object.spec.replicas <= 100", Message: "replicas must not exceed 100"},
      },
  }
  ev, _ := celpkg.NewEvaluator()
  res := pol.Check(ev, newDeployment("cel-demo", 2))

Proposed:

ev, _ := cel.NewEvaluator(cel.WithObjectBinding(newDeployment("cel-demo", 2)))
ev.AddExpression("object.spec.replicas >= 1",   "replicas must be at least 1")
ev.AddExpression("object.spec.replicas <= 100", "replicas must not exceed 100")
result, err := ev.Check()
if result.Passed(){ ... }

Let's start with that. Let me know what you think

[vladimirvivien]

See my PR comment.

[crandles]

I think I like your proposal -- no objections with the interface. The struct field options appears overly verbose, with no strong benefit.

Will push up changes shortly.