Port Rechnung-Adaptions to BO4E-Python

[hf-kklein]

• IstReverseCharge
• IstSelbstausgestellt
• Vorauszahlungen
• Sonderrechnungsart
• Buchungsdatum

Please open Pull Requests against https://github.com/bo4e/BO4E-python.
Each Pull Request should contain one atomic change.
Maybe first create issues where you describe the functional need for the adjustments. Align with @hf-mrdachner please