Add --exclude-system option to dumpdata to skip Django-managed models

[abdullah]

Currently, when using dumpdata to export application data, users often need to manually exclude Django-managed models such as content types, permissions, admin logs, and sessions.

For example:

python manage.py dumpdata \
  --exclude contenttypes \
  --exclude auth.Permission \
  --exclude admin.LogEntry \
  --exclude sessions.Session

This is repetitive and easy to get wrong, especially across projects with different INSTALLED_APPS.

Some of this data is environment-specific or regenerated by Django through migrations and post-migrate handlers. Including it in fixtures can cause conflicts or unwanted records when loading data into another database.

I would like to propose adding a convenience option to dumpdata, for example:

python manage.py dumpdata --exclude-system

or possibly:

python manage.py dumpdata --exclude-internal

This option would exclude a predefined set of Django-managed/internal models from serialization.

Possible candidates could include:

- contenttypes.ContentType
- auth.Permission
- admin.LogEntry
- sessions.Session

The exact list and option name can be discussed.

This would be backwards-compatible because the behavior would only apply when the option is explicitly provided. Existing dumpdata behavior would remain unchanged.