CDK8s+: Support headless services without ports

[sweco-sedalh]

Description of the bug:

We use a headless service (clusterIP: "None") just for the reverse DNS lookup (PTR), so have no need to specify ports. I imagine that that is a reasonably common usecase.

However, the current CDK8+ Service requires ports to be specified. Therefore I'd want that validation to be ignored when given a headless service.

An added bonus would be explicit support for headless services, by making the type of clusterIP be string | "None" | undefined or string | ServiceClusterIP | undefined (where ServiceClusterIP is an enum with a single value HEADLESS that resolves to "None" in the generated manifest).

Reproduction Steps:

Create a headless service:

new Service(this, "headless-svc", {
  clusterIP: "None",
  selector: deployment,
});

Error Log:

Error: Failed serializing construct at path 'app/traefik-headless-svc/Resource' with name 'traefik-headless': Error: A service must be configured with a port

Environment:

• Framework Version:
• OS:

Other:

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