[avalanche] Split the AvalancheProof class into legacy and regular in the test framework

Summary:
So we can use both formats in tests. Note that the tests still use the legacy format for now, which is the node default.

Ref T1676.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1676

Differential Revision: https://reviews.bitcoinabc.org/D10358