[avalanche] Add a LimitedProofId::computeProofId method

Summary:
The piece of proof.cpp that computes the proodif from the limited id and
from the master key will be needed when we verify that a delegation
matches a given proof. This adds a method to LimitedProofId to do the
computation.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9596