feat-impl: custom attributes for users

[rubenvitt]

... can't be tested well, so please double-check. I'll try to fix, if you find something.

[gmessner]

Haven't tested yet, but I believe this may fail if the authenticated user is not an administrator. https://docs.gitlab.com/ee/api/custom_attributes.html states the following:

"Every API call to custom attributes must be authenticated as administrator."

Also, I'm not a fan of adding "?with_custom_attributes=true" as part of the path, it should be moved to the query params that are being passed in.

I'm thinking it might be better to make custom attributes a property of the UserApi so that one could enable custom attributes by doing something like this:

gitLabApi.getUserApi().enableCustomAttributes();
List<User> users = gitLabApi.getUserApi().getUsers();

Or on a per call basis like this:

List<User> users = gitLabApi.getUserApi().withCustomAttributes().getUsers();

[gmessner]

@rubenvitt
The changes are very near exactly what I was looking for. If you are done I will go ahead and merge and then make a couple of small tweaks, let me know if it is ready.

[rubenvitt]

I think, that's all. :)
Thanks!