Add Support for Escrowed Checkpoint Signature Rewards

[karlem]

Problem

In a child subnet, 10 validators sign a checkpoint. However, as a relayer, I only need to submit 7 signatures to meet the quorum. To save costs, I omit the remaining 3 signatures. From the parent subnet’s perspective, only 7 validators appear to have participated, even though 10 actually did.

Alternatively, the relayer might submit the checkpoint as soon as the quorum is reached without waiting for additional signatures. This results in validators who signed later not receiving rewards for their participation.

Proposed Solution

Allow relayers to append additional signatures after submitting a checkpoint ID to the parent subnet. Relayers should have an economic incentive to do so.

To enable this, rewards for checkpoint signatures should be escrowed for a grace period. This gives the relayer time to gather and submit additional signatures, ensuring fair reward distribution among all signing validators.