[avalanche] Only sign the subset of data we want to commit the stakes to

Summary:
This makes it possible to update all the other parameters from a proof without having the stakers to sign again.

Ref T1676.

Depends on D10140.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, PiRK

Maniphest Tasks: T1676

Differential Revision: https://reviews.bitcoinabc.org/D10151