Using oci_image_index doesn't produce a .digest target while oci_image does

[mislavmandaricaxilis]

After #346 got merged, using oci_image macro also produces a .digest target, which contains a digest of the image.

But this was not done for oci_image_index, so switching to a multi-arch images causes a regression in experience because .digest target is missing.

Same as with oci_image, it's easy to write it ourselves, but having it in rules_oci directly makes it easy and consistent.

I'm willing to contribute a PR myself for this change if maintainers agree with the suggestion.