Add an `Authorize` activity?

**Issue**
While you can remove rights from resources or add read rights using WAC, this is not possible using ActivityPub. The only way, read rights are granted is through `Create` or `Update` activities by adding the recipients to the WAC read list.

**Proposal**
We could add a custom `Authorize` activity that lists actors which to grant or remove certain rights to / from.
Since this is non-standard it might be good, to keep in touch with the AP community.

No file, test, or entry point is named. Start by reviewing the repository's ActivityPub and WAC handling and checking the current ActivityPub community position on custom authorization activities; the work is done only when the proposed behavior and compatibility approach are agreed and documented.

Written by the indexing model from the issue text.