Support reading credential store from Buildah

[FeynmanZhou]

Is your feature request related to a problem?

The user would like to authenticate with a container registry after buildah login. Buildah generates an authentication file in ${XDG_RUNTIME_DIR}/containers/auth.json. Notation doesn't support reading credential store from Buildah authentication file.

What solution do you propose?

Support reading credential store from Buildah. If the user has Buildah installed on the system and used buildah login to authenticate with an OCI-compliant registry, the user's credentials are already stored and available to notation. In this case, the user doesn't need to run notation login again to authenticate with an OCI-compliant registry.

What alternatives have you considered?

N/A

Any additional context?

FYI: https://cloud-native.slack.com/archives/CQUH8U287/p1727891500206269