Allow adding/updating a license via the API

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/15602

The API currently only allows getting the current license. It would be handy to add or update a license through an endpoint, too. The use-case is for automation.

cc/ @JobV Should be pretty easy. If you think it's reasonable, please schedule.

Makes sense

Milestone changed to 8.8

Added promised label

Requested by: https://gitlab.zendesk.com/agent/tickets/23547

mentioned in merge request !400 (merged)

Reassigned to @dblessing

Status changed to closed by merge request !400 (merged)

mentioned in commit 26cfd3ee

Mentioned in commit 26cfd3ee