[avalanche] make delegateavalancheproof take a limited id rather than a whole proof

Summary:
The DelegationBuilder does not need a complete proof to build a delegation, only its master public key and its limited proof id. Add a constructor that does not require the proof.
Remove also the proofid member that can be computed from the limited ID.

Replace the DelegationBuilder::importDelegation method with a DelegationBuilder(const Delegation &dg) constructor.

Use these 2 new constructors in the delegateavalancheproof RPC.

Use a private constructor to factor the common code for the 3 public constructors.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix, Fabien

Subscribers: Fabien, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9613