Warn when a "fat" tar is provided as input

Since you've made it possible to pass an archive file as a layer, many rules_oci users will take the easy route of continuing their existing `tar.bzl` rules and pass them as image inputs. Of course these files are large and de-optimize the build. How will they know there's a better way to pass only mtree files instead?

Start by tracing how archive-file layers are accepted, including the existing tar.bzl and rules_oci paths mentioned in the issue. Determine how to distinguish a large tar input from an mtree input, then verify that the warning appears for fat tar archives while mtree-based inputs remain unaffected.

Written by the indexing model from the issue text.