Add an action to delete a billwerk customer, if the Contract ID listed in Drupal is the only contract

Problem/Motivation

Add an action to delete a billwerk customer, if the Contract ID listed in Drupal is the only contract:
https://docs.frisbii-transform.com/reference/customers_delete_id_delete

https://docs.frisbii-transform.com/reference/contracts_delete_id_delete
Note: For GDPR personal data deletion request, DELETE /customers/{id} is also available and will ensure all contracts are deleted.

This is for example needed, if the contract was created for a fake account or the user requested deletion.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes