[avalanche tests] implement AvalancheProof serialization

Summary:
This diff implements a proper serialization and deserialization for the
AvalancheProof object in the test framework, so we can inspect more in
details the data contained in the proof shared by a node and print a more
user-friendly representation of it.

Based on code from abandoned diff D9341.

Depends on D9360.

Test Plan:
ninja check-functional

The unit test added in D9360 ensures that a proof remains unchanged
after a serialization-deserialization round-trip.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9363