[avalanche] Make gen_proof facility return the key object

Summary:
There is currently a single callsite using the private key output, which is retuned as wif. Returning the key object makes it more versatile as the pubkey can be computed as needed as well. This will be used in an upcoming new test.

Ref T1634.

Test Plan:

ninja all check-all
./test/functional/test_runner.py abc_p2p_proof_inventory

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Maniphest Tasks: T1634

Differential Revision: https://reviews.bitcoinabc.org/D10026