Added the possibility to use custom labels instead of 'old' and 'new'

Summary

This is related to: caosdb-advanced-user-tools#77 (moved)

It introduces two arguments for describe_diff that allow setting custom labels instead of 'old' and 'new' in the difference string representation.

Furthermore, I added docstrings and type hints.

Test Environment

I have added a unit test to test the function (specifically the new feature).

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• All automated tests pass
• Reference related issues
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• Up-to-date JSON schema (or not necessary)
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  • Problems with old code
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For further good practices have a look at our review guidelines.