[avalanche] Add a verifyavalanchedelegation RPC

Summary:
This checks the delegation signature stack is correct.
Note that since it shares the error path with other RPCs which have full coverage already, I only added a happy path test to avoid needless redundancy.

Depends on D11236.

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/D11237