[avalanche] Decode the payout script and proof signature in the decodeavalancheproof RPC

Summary:
This is only available when the legacy format is not used.

Ref T1676.

Depends on D10358.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1676

Differential Revision: https://reviews.bitcoinabc.org/D10359