RFC: New license attribute for license text/expression addition

There are licenses which are `is_exception`: `True` and these actually should be paired with the licenses they are an exception to,
in the case of generating attribution and also while getting the resultant license expression if detected.
See https://github.com/nexB/scancode-toolkit/blob/develop/src/licensedcode/data/licenses/openssl-exception-gpl-2.0.LICENSE for an example. We can implement this by adding a new license attribute which points to the license key which this exception is related to. This will be a list btw as there can be licenses without versions, but we have to see which one of these to include for text/expressions.

Suggestions:
- `exception_to`
- `include_text_from`

Also similar issue, but a bit different: https://github.com/nexB/scancode-toolkit/issues/3248#issuecomment-1448062797