allow tag-less promotions (promote images without tagging them)

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Today the promoter will only promote those images that have tags in them. This feature will allow images to be promoted even if they don't have tags.

We could easily shell out to https://github.com/google/go-containerregistry/tree/master/cmd/gcrane#cp instead of the usual "gcloud container add-tag" command we use; the only annoying part here is that we would have to ship the gcrane binaries from the offiical gcr.io/go-containerregistry/gcrane path, but it should be doable. Alternatively, we could drop our reliance on gcloud container add-tag altogether and just lift the code already in gcrane to use the underlying API calls to GCR (we already import the go-containerregistry library that gcrane uses for other parts of our codebase). This latter approach is the better but obviously it might take more engineering effort.

This issue is a blocker for creating the backfilled promoter manifest for gcr.io/google-containers. That is, we want to have a (large) promoter manifest for all images in gcr.io/google-containers.

We already support the -snapshot flag which allows for snapshotting a registry (all images, including both tagged and tagless images); so this feature would allow for promotions of said snapshots as-is.

[listx]

Cross-reference links: #70 #118