Add B2B Debit: HKBME/HKBSE

[saschnet]

Currently, only SEPA-Lastschriften (CORE) are supported through HKDMC and HKDME.

It would be helpful to also add Firmen-Lastschriften (B2B):
HKBME – Terminierte SEPA-Firmensammellastschrift einreichen
HKBSE – Terminierte SEPA-Firmeneinzellastschrift einreichen

The XML itself can in turn be created as documented at https://python-fints.readthedocs.io/en/latest/debits.html by the user in is outside of the scope of the package.

Thanks a lot for the work!