Feature Request: Update Transaction POST API to support Transfers

[minideezel]

Describe the feature

Support posting a transaction to the api that has both the source and destination account and comes in as a transfer.

Why is this feature important?

I would like to move my 10 years of transactions from YNAB / Actual-Budget into Sure, but in order to have accurate history I need to be able to import transfers between accounts and there's currently no way to do that via import (requested in #190 ) or API.

Additional context, screenshots, and relevant links

No response