[avalanche] Add a decodeavalanchedelegation RPC
Summary: Can be used to check the content of a delegation and retrieve the associated proof id.
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/D11236