Add support for username update

[Konig-Corey]

Api documentation shows an api for "/users/rename" to update a username of a user. The current update function is only for updating non username fields. We should add a new method "rename" under admin.users.update. that takes in the old username and the new username.

API Docs: https://docs.atlassian.com/bitbucket-server/rest/5.7.0/bitbucket-rest.html#idm45568366848400
Api path: /rest/api/1.0/admin/users/rename