[avalanche] Implement proof expiration time

Summary: A proof with a negative or zero expiration time will never expire, any other value will expire once the tip MTP is greater or equal to the expiration time.

Test Plan:

ninja all check-extended

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11873