text: Title: Introduce a global setting to always get all element from `list()` calls
  Body: ## Description of the problem
  
  In the gitlabform app (https://github.com/gitlabform/gitlabform), and probably for many other purposes too, we always want to get all the elements of lists.
  
  ## Current solution
  
  There seem to be 2 options for that when using this library:
  
  a) remember to use `get_all=True` and probably cause with increased resources use on the GitLab instance,
  
  b) remember to use `iterator=True` and add extra code to get all the elements of the list,
  
  Both of these require extra code that you need to remember about (because the missing parameter will not cause an error unless you test with lists of > 20 elements) and are inconvenient.
  
  We already had 2 bugs in gitlabform because we forgot to add this code:
  * https://github.com/gitlabform/gitlabform/issues/732 , PR: https://github.com/gitlabform/gitlabform/pull/735
  * https://github.com/gitlabform/gitlabform/pull/731
  
  ## Proposed solution
  
  Let's introduce a global configuration flag to get all the elements of each list by default.
  
  We could name this flag `get_all_by_default` and pass it to the constructor ([like the "pagination" parameter now](https://python-gitlab.readthedocs.io/en/v4.4.0/api-usage.html#pagination)).
  
  This could be implemented with iterators to not cause an extra load in the GitLab instance. The execution time or resource use on the client side is less important here as this would probably be used for scripting, batch operations and generally non-interactive purposes.
  
  ## Specifications
  
    - python-gitlab version: v4.4.0
    - API version you are using (v3/v4): v4
    - Gitlab server version (or gitlab.com): doesn't matter