VC: Publish to multiple BNs

[lfinbob]

I have my lighthouse VC connected to 2 BNs (primary and fallback). I propose to add a flag that makes the VC publish messages via all connected BNs concurrently. That would sidestep bad fall-back experiences (see comment in 3002) with missed attestions/proposals/sync committees. It would also increase the chances of messages being successfully published despite spotty internet connections with occasional packet loss on the beacon nodes.

Implementing this would make #3001 and #3002 not required anymore for solid fall-back.