Configure `BeaconNodeFallback` via CLI

[dknopik]

The BeaconNodeFallback struct from LH provides a way to connect to multiple Beacon Nodes. It has two parameters that can be configured beyond the BN URLs:

• broadcast_topics: Which requests should be broadcasted to all BNs instead of sending sequentially, stopping with the first successful request.
• sync_tolerances: Which nodes do we consider synced, based on distance between current slot and current head?

We currently use the default values only. Power users might want to set these values via CLI, as is possible with Lighthouses CLI. Implement this, reusing CLI parameter names from lighthouse.

[0xriazaka]

can i work on that?

[Zacholme7]

@0xriazaka Yep, let one of us know if you have any issues/questions!