Add `additional_info` attribute to `license` rule.

## Expected Behavior

As the [design doc](https://docs.google.com/document/d/1uwBuhAoBNrw8tmFs-NxlssI6VRolidGYdYqagLqHWt8) says,
> `additional_info`: a map of strings to strings. This is a catch-all bucket for users to add custom metadata about this license instance. For example, if this is a paid license, it might indicate if it is site-wide or limited to a number of simultaneous instances. Nothing in this document refers to additional_info again.

## Actual Behavior

No such attribute on the rule.

Start by reading the linked design document and locating the `license` rule in the repository. Add the `additional_info` attribute as a map of strings to strings, and confirm that the rule exposes the requested custom metadata.

Written by the indexing model from the issue text.