[avalanche] Add manual stake winners to the contender cache

Summary:
This adds manual winners to the contender cache when the setstakingreward RPC is called.

This patch incorporates early feedback from D16992

Test Plan:

ninja check-avalanche-processor_tests

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17117