[avalanche] Introduce StakeContenderId

Summary: StakeContenderId will enable the node to poll for potential stake winners (as a new CInv type). This will further enable pre-consensus on staking rewards to ensure that nodes validate the same winner set even if some proofs are missing from some nodes' proof sets.

Test Plan:

ninja check-avalanche

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16631