Changing validator client list of beacon node without restart

[jakubgs]

Is your feature request related to a problem? Please describe.
Changing the list of used beacon nodes is a common administrative task when managing nodes. For example when a BN or EL exhibit issues and need to be taken offline for maintenance.

Currently the only way to do this is to restart the VC, which will cause downtime every time, especially if Doppelganger Detection is enabled.

Describe the solution you'd like
One way to achieve this would be to allow for reloading of config file, either via a signal or an API call.

Describe alternatives you've considered
Another way that's more beacon node specific would be to send a list of endpoints to the VC via the API.