New devtool to update testdata

[david-venhoff]

Motivation

Starting from #2781 we have a new workflow for updating the testdata file. To make it as easy as possible for developers to update it in case of further changes, we should add a new devtool that updates the testdata file based on the current database. We also need some documentation.

Proposed Solution

Add a new devtool that behaves roughly like this command: ./tools/integreat-cms-cli dumpdata --natural-primary --natural-foreign --exclude auth --exclude contenttypes --o test_data.json

Alternatives

Do nothing, maybe just document what command should be used to update the test data

User Story

As a developer I want to quickly update the testdata when writing tests.

Additional Context

Design Requirements