Split the principal into multiple parties when they contribute to funding a validator

🎯 Problem to be solved

When multiple parties contribute to funding a validator, it's not currently possible to split the principal into multiple parties. We should mimic the behaviour we have now for rewardAddress on principalAddress, ensuring safety and minimizing the risk of losing funds.

🛠️ Proposed solution

• Investigate how to allow splitting the principal between different parties when funding a validator, considering the safety concerns and potential risks involved.
• Determine the feasibility of implementing such a feature within the existing Splits contract architecture while maintaining high security and reliability.
• Design and document the proposed solution, including contract modifications and user flows, with a focus on ensuring that the process is foolproof and minimizes the risk of losing funds.

This proposed solution aims to provide more flexibility and options for validators funded by multiple parties, allowing them to distribute the principal according to their specific needs and agreements.

No files or tests are named. Start by tracing the existing rewardAddress splitting behavior and the principalAddress handling in the Splits contract architecture; document a safe design, contract changes, and user flows that address the stated fund-loss risks.