User Account: Add authclient, authclient_id.

[luke-]

• It should be possible to specify the following optional account fields when creating a user:

• authclient referes to user.auth_mode

• authclient_id referes to user.authclient_id.

https://github.com/humhub/rest/blob/master/definitions/UserDefinitions.php#L51

These are also to be returned.

• In addition to /user/get-by-username, there should also be a /user/get-by-authclient.

This should find the user by the parameters name of the authclient (user.auth_mode) and id value of user.authclient_id.

[yurabakhtin]

@luke- PR #111: Please note on previous version the core scenario User::SCENARIO_EDIT_ADMIN was used from REST API side for add and update user, in order to don't touch the core scenario I decided to extend the model User from REST module side with new model ApiUser.
If you don't like such wrapper/extenssion please let me know, and otherwise we should modify a core side in order to either add fields into the existing scenario or create new one with name like SCENARIO_API_REQUEST = 'apiRequest, but I am not sure it is a good idea to mix a code from module with core code.

[luke-]

@yurabakhtin Thanks, the Wrapper Solution is good in this case.