[avalanche] Handle tx inv polls with a stub handler.

Summary:
Changes the AVAPOLL message handler to recognize MSG_TX invs and respond to them with votes based on a helper function in the same pattern as we do for block and proof polls.

The actual implementation of the helper function is left for a follow-on revision.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_transaction_voting.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11314