[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