[avalanche] Send proof inventories
Add the proof inventories in the inv messages. For now the proofs are
only added manually via the sendavalancheproof RPC.
This is heavily based on the current implementation for transactions.
Depends on D9654.
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Maniphest Tasks: T1611
Differential Revision: https://reviews.bitcoinabc.org/D9650