[avalanche] Add buildavalanchproof RPC call

Summary: As per title. This RPC allow to generate proof from a JSON description of it.

Test Plan:

ninja all check-all

The integration test has been updated to use the RPC in question.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6914