Store basic auth credentials in Secret for Image with blob source.

[JMesens]

Currently I pass the basic auth credentials in the blob-url of an Image.

        apiVersion: kpack.io/v1alpha1
        kind: Image
        metadata:
          name: image-name
        spec:
          source:
            blob:
              url: https://user:longsupersecurepassword@maven/v1/group/version/artifact-version.jar

It would be nice if the user:longsupersecurepassword credentials for basic auth also could be configured through a Secret so they don't have to be passed in the url.