text: Title: Add application statistics
  Body: ## Description of the problem, including code/CLI snippet
  
  The Gitlab provides some general statistics via `/application/statistics`.
  See https://docs.gitlab.com/ee/api/statistics.html
  
  ## Expected Behavior
  
  These data are available via `gitlab.statistics()`.
  
  ## Actual Behavior
  
  Nothing.
  
  ## Specifications
  
    - python-gitlab version:
    - API version you are using (v3/v4):
    - Gitlab server version (or gitlab.com):
  