VC option to pass subscriptions to fallback nodes

[paulhauner]

Description

Following from this thread, it would be nice if the VC had an opt-in flag to publish subscriptions to all backup endpoints (or maybe even specific endpoints :thinking:). This means that backup BNs don't need to run with subscribe-all-subnets to be reliable.

I'm cautious of incurring usage on third-party nodes, that's why I suggested opt-in.

[lfinbob]

I'm very much in favor of this. With the post-merge reality setting in we'll have a lot of staking enthusiasts using a simple fall back EL+CL. Rocket Pool makes this dead simple for example. They will assume that a fall back beacon node works automatically and without missing attestations (proposals, sync commitees). But it doesn't. Unless you specify --subscribe-all-subnets which is wasteful to home stakers' bandwidth.

[maninthecryptosuit]

I'm also in favour of this, it would help significantly reduce CPU, RAM, bandwidth and power usage on the backup node.

[michaelsproul]

Implemented in https://github.com/sigp/lighthouse/pull/3529 and will be part of the next release 🎉