[avalanche] Add a RPC to check if a block/tx is finalized by avalanche

Summary: This will make it easier for exchanges to implement 1-block confirmation.

Test Plan:
With clang and debug:

ninja all check-all

Run the ASAN and TSAN builds.

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11996