code: func (c *Client) AuthenticateWebUser(req AuthenticateUserRequest) (services.WebSession, error) {
  	out, err := c.PostJSON(
  		c.Endpoint("users", req.Username, "web", "authenticate"),
  		req,
  	)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	return services.GetWebSessionMarshaler().UnmarshalWebSession(out.Bytes())
  }
docs: // AuthenticateWebUser authenticates web user, creates and  returns web session
  // in case if authentication is successful