[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