Investigate what to have as image build output (jib-image.json, jib-image.digest, and jib-image.id) for multi-platform image building

Should we write multiple `jib-image.digest` and `jib-image.id` files for each manifest? What about the file names? Should we also write a digest of a manifest list (potentially with a different name)? Or, should we write only a single digest for a manifest list?

Does it make sense change the JSON format of `jib-image.json`? Should we keep backward compatibility?

On the technical side, we need to figure out the right return type of `PushImageStep` after sorting out these questions:

https://github.com/GoogleContainerTools/jib/blob/94f8bdafe0f816c548f96392b34660d5574cc5c4/jib-core/src/main/java/com/google/cloud/tools/jib/builder/steps/PushImageStep.java#L40-L46