text: Title: Add Merge Request merge_status and detailed_merge_status constants
  Body: ## Description of the problem, including code/CLI snippet
  
  There are a bunch of possible `merge_status` and `detailed_merge_status` values for a Merge Request. Add these as constants.
  
  
  ## Expected Behavior
  
  You can find the possible values in the `const` module.
  
  
  ## Actual Behavior
  
  The values are not to be found in the library.
  
  
  ## Specifications
  
    - python-gitlab version: 3.15.0
    - API version you are using (v3/v4): v4
    - Gitlab server version (or gitlab.com): n/a
  