feat: add docker_registry_multiarch_image data source

[vprakash-seatgeek]

Added a new docker_registry_multiarch_image data source to expose details about Docker multi-arch images. When you build a multi-arch image, the underlying images are not tagged so it is useful to be able to look up details about them.

In the process, also refactored some auth helper methods out from internal/provider/data_source_docker_registry_image.go to reduce code duplication.

[komapa]

This will be great if we can upstream instead of having to fork the provider.