Notes on TAN handling

[barredterra]

TAN handling is not implemented yet. Here are some notes how it could be built.

1. When a TAN is required, the state of the FinTS3PinTanClient needs to be stored on the server / database (for example, in the FinTS Login doctype).
2. We send a note to the frontend saying that a TAN is required (frappe.publish_realtime('tan_response_needed'))
3. The frontend asks the user for the TAN (frappe.prompt()).
4. The frontend sends the TAN to a whitelisted method in the backend.
5. The backend method restores the previous state and sends the TAN to the bank.

Relevant Docs:

• Storing and restoring dialog state
• Storing and restoring TAN state

[jHetzer]

I will dig into it when I have time. PR's into develop are welcome

[jHetzer]

Unfortunately i looked my online banking and cannot proceed with programing. Waiting for reactivation. PR's into develop are welcome.