[avalanche] Add a finalizetransaction RPC to force finalize a mempool transaction

Summary:
This is a risky RPC as it could lead to a different status than the network and a rejected block, but it can be used to include a transaction in a block after preconsensus is active.

Depends on D18791.

Test Plan:

./test/functional/test_runner.py abc_rpc_finalizetransaction

Reviewers: #bitcoin_abc, bytesofman, PiRK

Reviewed By: #bitcoin_abc, bytesofman, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18787