List of String type enforcement and default

[cueckoo]

Originally opened by @ekarlso in cuelang/cue#400

Can I somehow specify foo: []string | *[commonName]

Usecase
We want to specify something like:

certificate: [ID=_]: cm_alpha3.#Certificate & {
	kind:       cm_alpha3.#CertificateKind
	apiVersion: "cert-manager.io/v1alpha3"
	metadata: name: ID

	spec: issuerRef: {
		name: string | *"letsencrypt-prod"
		kind: string | *cm_alpha3.#ClusterIssuerKind
	}

	spec: {
		secretName: string | *ID
		commonName: string | <5
        renewBefore: string | *"360h0m0s"
        dnsNames: string | * [commonName]
	}
}

And have the dnsNames be default to [commonName] if dnsNames is not passed. Is this doable?

[cueckoo]

Original reply by @gogommartin in cuelang/cue#400 (comment)

I'm new to Cue myself so this may not be correct, but I believe what you're looking for is:

dnsNames: [...string] | *[commonName]