Use annotations to map a zTOC to its corresponding layer

[wmesard]

Is your feature request related to a problem? Please describe.

For very small layers, it’s not worth creating a zTOC; it’s more performant to just pull the layer at launch time, and untar it. Hence, the soci create --min-layer-size option.

The problem is that today we use null values in the descriptors array to represent these "missing" zTOCs.
In other words, soci.descriptors[i] always corresponds to image.layers[i]. The problem is that ORAS doesn't like that.
This wasn't necessarily a deal breaker, since we will soon be moving to Reference Types. But, they don't really like it either.

Describe the solution you'd like

Rather than fight this fight, let's just go with the annotation based approach. That is, each entry in the descriptors array can have an annotation containing the digest of the corresponding layer.