Feature request: add mirroring support

It seems one can only specify a single URL in the [lock file](https://github.com/bazel-contrib/rules_multitool/blob/main/lockfile.schema.json). So the tools are always fetched from the internet. I'd really prefer mirroring the files and archives on a CI-internal server and only have the internet as a fallback.
I think an API similar to that of the BCR would be applicable here as well. That is, adding an optional [`mirror_urls` field](https://bazel.build/external/registry#source-json) to the lock file, or a global, top-level [`mirrors` field](https://bazel.build/external/registry#bazel-registry-json), or both.

Have you considered something like this and would you be interested in the feature?