[avalanche] Retrieve the acceptance status and confidence of the proof vote records

Summary:
This extends the processor's isAccepted() and getConfidence() API to proofs.

Ref T1854.

Test Plan:

ninja check-avalanche-processor_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10269