Add new endpoints for License Management (Enterprise Edition)

[DiscoPYF]

https://www.arangodb.com/docs/3.9/release-notes-api-changes39.html#license-management-enterprise-edition

Two endpoints were added for the new License Management. They can be called on single servers, Coordinators and DB-Servers: GET /_admin/license PUT /_admin/license

The route starts with _admin, same as a bunch of other endpoints in Miscellaneous functions and Administration and Monitoring. We could create an AdminApiClient that expose all these endpoints.