Extract user UID/GID injection functionality to its own class

## Motivation

- The methods `get_overriding_uid()`, `get_overriding_gid()`, `get_supplementary_gids()` in `AbstractAgent` don’t really belong in the definition of AbstractAgent as it involves a concrete functionality rather than a more abstract function

## Required Features

- Move the functionality of injecting overridden uid/gid into its own separate class/module

## Impact

- Should not cause any regression from existing behavior

## Testing Scenarios

- Should not cause any regression from existing behavior

JIRA Issue: BA-3073