Add Duniter backend
Should not be too hard to implement, modulo things...
Documentation:
https://git.duniter.org/nodes/typescript/duniter/blob/dev/doc/Protocol.md#transaction
Explicit transactions: https://git.duniter.org/nodes/typescript/duniter/blob/dev/doc/HTTP_API.md#txhistorypubkey UD: https://git.duniter.org/nodes/typescript/duniter/blob/dev/doc/HTTP_API.md#ud