Payjoin V2 example

[aagbotemi]

Description

This PR provides an example of using Payjoin with bdk-tx. This PR fixes #3

Note to the reviewers

Currently, WIP. Finalizing PSBT for the receiver. A change output is being created resulting in 2 outputs.

Key components:

• Payjoin session initialization.
• Demonstrates PSBT building by sender
• Receiver inputs contribution with privacy and validation check
• PSBT finalization logic for both sender and receiver wallets

All Submissions:

• I've signed all my commits
• I followed the conventional commit guidelines
• I ran cargo +nigthly fmt before committing