[avalanche] implement buildRemoteSighash and buildLocalSighash

Summary: buildLocalSighash is extracted from sendHello and buildRemoteSighash will be used in D9327 to check the AVAHELLO message sent by a peer.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9329