Add possibility to customize the completion script

Currently the completion script is auto-generated from ArgumentParser and cannot be modified.
In some (rare) projects this auto-generated completion script can be unadapted. We should be able to customize it to our particular needs.

Start by reading how ArgumentParser currently generates the completion script and identify where customization would need to enter the process. Define the intended customization mechanism and its boundaries for projects whose generated script is unsuitable, then verify that the resulting completion output can still be generated and used.

Written by the indexing model from the issue text.