[avalanche] Add stake contender inv type

Summary: This patch adds the stake contender inv type as a necessary precursor to finalizing stake winners in pre-consensus. This inv type is not actually requestable, so it is ignored. It will be used to poll for stake winners in a future patch.

Test Plan:

ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, PiRK

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16668