GCP auth per target and source

[arielt17]

Hi, i would like to use the tool to sync between two registries that i have on GCP, the issue is each of them sits in a different project.
An option to set the authentication configuration for each of them would allow me two authenticate separately to each project with the least privileges.

[art-shutter]

that could be achieved by not relying on the auth in dregsy, but config for skopeo or docker.
For docker it is enough to run gcloud auth configure-docker whatever-docker.pkg.dev,whatnot-docker.pkg.dev
For skopeo you would also need to copy the credHelpers section that appears after running configure-docker in ~/.docker/config.json to ~/.config/containers/auth.json

[xelalexv]

Ignore above commit, had wrong issue number in commit comment.