Provide a way of packaging separate layers into Java container after deprecation of rules_docker

(It feels awkward to ask this question here, but it is a follow-up of [ this suggestion over at rules_oci ](https://github.com/bazel-contrib/rules_oci/issues/416#issuecomment-1832829593))

As the maintainers have decided that the rules_oci repository should remain language agnostic, it is quite unclear where to go with questions for language-specific containers, so following @alexeagle 's advice I am trying over here.

I would like to migrate from rules_docker to rules_oci, but the functionality to package depsets into separate layers has not been provided by the new rules ([_jar_app_layer_impl](https://github.com/bazelbuild/rules_docker/blob/8e70c6bcb584a15a8fd061ea489b933c0ff344ca/java/image.bzl#L178-L212) ).

Would it be possible to provide similar functionality in a `.defs` file in this repository, and could someone help me out in porting it?

Start by reading the referenced _jar_app_layer_impl in rules_docker's java/image.bzl and the linked rules_oci discussion. Determine the intended home and scope for equivalent depset-to-layer packaging in this repository. Done means the requested Java container layering functionality is provided through a .defs file with a clear migration path from rules_docker.

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