Check if user exists endpoint

[BrianSetz]

I may have overlooked it, but it looks like the endpoint https://gitlab.com/users/:username/exists is not implemented?

Example: https://gitlab.com/users/example/exists

Which returns:

{ "exists":true }

If it is indeed not available in gitlab4j, would there be an interest in adding this functionality?

[jmini]

This seems to be missing