WIP Payjoin upgrade to native dart bindings via payjoin_ffi

[benalleng]

Closes #3173

Description

This is a WIP change to use native dart bindings in payjoin_ffi

TODO:

• figure out final psbt receiver signing error
  • constantly getting this error on the sender side.

Details

  Error: Failed to broadcast transaction
Caused by:
    RPC server returned error 'mempool-script-verify-flag-failed (Signature must be zero for failed 
CHECK(MULTI)SIG operation), input 1 of 78518ef0206660d349070be038c65c50e22dae0255bd763143228d3b05eac
10d (wtxid c5ca945f1e92c57077c21373242d9946c9bfd460beb79d8816a31fc03aa2458a), spending d7ed58ebaf291
38c203b61536bdea358b8b3f4173b1e9a6cecf61154d540b380:0' (code -26)

• Ensure there are some initial guards as payjoin sessions are created automatically in the UI here
  • guard when there are no available utxos to build from to not start a session that will inevitably fail
• Create better session cancel behavior
• update relay and directory selection to follow best practices
• use a more up-to-date bindings publish as we have made some changes since 0.1.1

Pull Request - Checklist

• Initial Manual Tests Passed
• Double check modified code and verify it with the feature/task requirements
• Format code
• Look for code duplication
• Clear naming for variables and methods
• Manual tests in accessibility mode (TalkBack on Android) passed