[avalanche] Add the score and staked amount to the decodeavalancheproof RPC

Summary: The missing score is certainly an oversight and the staked amount a convenience to avoid summing all the stake amounts.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11142