[avalanche] Add a mechanism to delegate proofs

Summary:
This is pretty much self explainatory. One does not want to spread its private keys around, so a mechanism to delagate a proof to any public key of the prover's chosing is necessary.

Depends on D8599

Test Plan: Added unit test.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8602