code: public User getUser(String username) throws GitLabApiException {
          GitLabApiForm formData = createGitLabApiForm().withParam("username", username, true);
          Response response = get(Response.Status.OK, formData.asMap(), "users");
          List<User> users = response.readEntity(new GenericType<List<User>>() {});
          return (users.isEmpty() ? null : users.get(0));
      }
docs: Lookup a user by username.  Returns null if not found.
  
  <p>NOTE: This is for admin users only.</p>
  
  <pre><code>GitLab Endpoint: GET /users?username=:username</code></pre>
  
  @param username the username of the user to get
  @return the User instance for the specified username, or null if not found
  @throws GitLabApiException if any exception occurs