[avalanche] Add a payout script to the proof
This will be used for stake rewards. Since it's changing the proof format, I hid it behind a flag so I don't have to update all the tests, especially since more format changes are coming (and updating the hardcoded proofs in tests is expensive).
The payout script cannot be changed without signing the stakes again. This will change in the future.
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Maniphest Tasks: T1676
Differential Revision: https://reviews.bitcoinabc.org/D10109