[avalanche] Add an option to import a delegation

Summary:
The "-avadelegation" option let the user supply an already built
delegation to the node, so the proof can be used without passing the
master private key around.

Depends on D9445.

Ref T1635.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1635

Differential Revision: https://reviews.bitcoinabc.org/D9365