Add flag to resolve platform-specific images

If a tag points to an [image index](https://github.com/opencontainers/image-spec/blob/master/image-index.md#oci-image-index-specification) or [manifest list](https://docs.docker.com/registry/spec/manifest-v2-2/#manifest-list), digester resolves the tag to the digest of the image index or manifest list.

This enhancement adds a `--platform` flag so that digester instead resolves the tag to the digest of the platform-specific image (e.g., `linux/amd64`).