[avalanche] Add a facility to build a delegation from a hex string

Summary:
Similar to what is done for proofs and others, this will be useful when
inputting a delegation from the command line.

Ref T1635.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1635

Differential Revision: https://reviews.bitcoinabc.org/D9664