Add managed license resources and tests

Created by: PatrickRice-KSC

I've updated the ResourceMap and created the resources for the gitlab_managed_license resources, matching the GitLab Managed License API Documentation. The tests themselves are flagged as requiring an EE license, since managed licenses are locked behind the ultimate license level.

While I'm not able to run the EE tests locally, I did run the provider/resource locally against a gitlab.com public project, and the resources appropriately created and destroyed licenses, so I believe the tests are ready to run against an EE instance.

closes #685 (closed)