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"
}