Add image pull secret generator to Timoni's CUE schemas

[stefanprodan]

Add generator for Kubernetes Secrets of type kubernetes.io/dockerconfigjson.

Example:

secret: timoniv1.#ImagePullSecret & {
	metadata: _config.metadata
	registry: "ghcr.io"
	username: "bot"
	password: "ghp_1234567890"
	suffix:   "-regcred"
}