[avalanche] Add getavalancheproofs RPC to retrieve tracked proof ids

Summary:
Useful for debugging and for node operators during setup and evaluating avalanche node health.
Adding this RPC removes the time consuming step of digging through the logs for proofids in
order to evalutate their statuses (valid, conflicting, immature).

Test Plan:

ninja
./test/functional/test_runner.py abc_rpc_getavalancheproofs

./bitcoind -daemon -testnet
./bitcoin-cli -testnet getavalancheinfo

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12062