[avalanche] Supply an optional delegation to the addavalanchenode RPC

Summary:
This makes it possible to manually add a node that doesn't have access to the proof master private key for signing the messages.

Ref T1635.

Test Plan:

./test/functional/test_runner.py abc_rpc_addavalanchenode

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1635

Differential Revision: https://reviews.bitcoinabc.org/D9792