[avalanche] Add a method to return if the node has a finalized tip

Summary: This method will be helpful to quickly check if the node is capable of polling without requiring all the quorum checks nor holding cs_main.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, PiRK, roqqit

Reviewed By: #bitcoin_abc, PiRK, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D16593