Simulate "postmix" tools - Fake coinjoins and send with multiple hops

[openoms]

The JoinMarket sendpayment.py is already capable of paying an arbitrary amount with a coinjoin with multiple participants to any address (SEND option).

Can simulate the SW Stonewall function with:
https://github.com/kristapsk/bitcoin-scripts
fake-coinjoin.sh

and the Ricochet (multiple hops) with:
https://github.com/kristapsk/bitcoin-scripts ricochet-send.sh

[kristapsk]

I guess currently biggest problem with my scripts is that they are written for Core wallet, but here this functionality is needed for JM wallet, where Core wallet is watchonly and so can't sign. One solution that just came in my mind is to write HWI compatible wrapper for JM and then add it to Bitcoin Core config as external signer. Additional care then should be taken also to not accidentlly mix UTXOs from different JM mixdepths, as Core and my scripts does not know anything about them.

I have in a longterm TODO list (in my head) some abstraction layer, so that these scripts could be used with Electrum, Wasabi and other wallets too (see kristapsk/bitcoin-scripts#7 (comment)), but that is more complicated solution as the one above.