Payjoin

[J0J0XMR]

This PR is a WIP.

The current bottle neck is that the PSBT being generated throws an error in internal PSBT data structure.

Cake uses PSBTv2 and Payjoin requires v0, so a conversion script is run after building the PSBT.
Testing the conversion script on a test PSBTv2 gives a valid v0, so the issue in how the v2 is created in electrum_wallet.dart using createPayjoinTransaction.

Raw hex, PSBT v0 & v2 strings are all printed in debug console for analysis.

Steps to reproduce:

1. Create a PjURI by choosing "Payjoin" option from the drop-down menu on the Receive screen.
2. Copy that PjURI and paste it in the Send form and press Send.

Note: Commits 25ae7c4 & 59ca8b1 include temporary deps and files needed to build and run this branch:

• bitcoin_base & payjoin_flutter packages are on temporary forked repos until the fixes get included upstream.
• bitcoin.dart is ignored by git and is needed to build, and will need to be updated again when merging to main.