FR: Can't use multiple labels file in single `rules_oci` rule

I want to add two kind of labels:
- labels for image lifecycle management - on this labels change image must be rebuild;
- labels for image stamping (from which Git revision I can build same image) - this lables change everytime and should be excluded from cache key.

For this I want to create two labels file and pass it by custom or `filegroup` rule:
- `labels.txt` - with labels for image lifecycle management;
- `labels_volatile.txt` - with stamping labels.

After that I can add `labels_volatile.txt` to `omitted_inputs` srubbing rule and cache will ignore every `labels_volatile.txt` file change.

But currently I can pass only single file by `labels` attribute to `rules_oci`.

Start by inspecting the rules_oci labels attribute and the omitted_inputs behavior described in the issue. Verify how labels.txt is currently accepted, then determine how labels.txt and labels_volatile.txt should be handled so volatile labels do not affect the cache key while lifecycle labels still do.

Written by the indexing model from the issue text.