[avalanche] Specify the proof to be used by the node via config

Summary:
As per title. Unused so far.

Depends on D8604

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8607