[avalanche] Decode the stake address in the decodeavalancheproof RPC

Summary: As per title, this comes in handy for doing lookups on explorers.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11139