[avalanche] Add an RPC to build a delegation

Summary:
This RPC makes it possible to build a delegation for a proof and stack
up delegation levels.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK, Mengerian

Reviewed By: #bitcoin_abc, PiRK, Mengerian

Subscribers: Mengerian, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9353