Allow producing Bazel-tracked layer blob on demand when compact layers are enabled

Currently, when compact layers are enabled, rules_img refuses to produce an oci layout for images that include such a layer.
Add a new flag that:
- allows using compact layers in oci layout and similar downstream artifacts
- adds a new action into all layer rules that will provide the `File` in the `blob` field of SingleLayerInfo

This means: the new action is only run if any downstream consumer needs it.
We should document the cost of this clearly.

[Slack thread](https://bazelbuild.slack.com/archives/CA3NW13MH/p1785961379031419?thread_ts=1782772343.038829&channel=CA3NW13MH&message_ts=1785961379.031419)

Start by tracing the layer rules and SingleLayerInfo, then follow how compact layers are rejected for an OCI layout and how downstream artifacts request layer data. Define the new flag and lazy blob-producing action across the layer rules, and document its cost; done means compact layers work for OCI and similar consumers without running the action unnecessarily.

Written by the indexing model from the issue text.