Implement Fallback Beacon Node Selection for Charon

[boulder225]

🎯 Problem to be solved

Charon currently doesn't have a reliable way to handle beacon node failures, which could lead to interruptions. We need to improve Charon's ability to switch to backup beacon nodes when the primary node fails.

🛠️ Proposed solution

• Implement a "pick list" feature in Charon for fallback beacon nodes
• Allow users to specify a list of primary and fallback beacon nodes through configuration
• When a call to a primary beacon node fails, automatically try the next available node from the fallback list
• If all nodes in the list fail, report the failure to Charon
• Ensure this feature works with the existing "multi" component that handles multiple beacon node connections
• Add error handling and logging for fallback node selection
• Update documentation to explain how to use and configure the fallback beacon node feature
• Create tests to verify the fallback mechanism works as expected