Add a new Tx method to settle challenge

[h3lio5]

Currently, the settling a challenge, i.e., determining the winner of the challenge, happens inside the EndBlock hook defined in the challenge module. We want to move the logic of determining winner and dispersing the reward into a new Tx method settleChallenge().

relevant codebase: https://github.com/topology-foundation/topchain-node/blob/main/x/challenge/module/module.go#L157