Track additional external validators in lodestar

@tuyennhv If I understand the goal is to track additional validators outside of the regular keys we run. To achieve the same we could modify the beacon node to:
- Provide a list of indexes to track as a BeaconNodeOptions argument. This indexes will be always tracked regardless of calls to prepareBeaconCommitteeSubnet. Then the beacon cli arg could read this indexes from a CLI arg, or from a file.
- Add a new debug or lodestar API endpoint where you pass a list of indexes and a number of epochs to track them, including infinite. We can also add another endpoint to remove indexes.

This approaches will prevent having to modify the validator package. Also since it's very simple to trigger then we won't need another CLI arg. The current name is a bit confusing tho since it's really a "trigger validatorMonitor" instead of just "validatorMonitor"

_Originally posted by @dapplion in https://github.com/ChainSafe/lodestar/pull/3964#issuecomment-1117607542_