Use validator/blocks v3 endpoint in the VC

[jimmygchen]

Description

The validator/blocks v3 implementation is currently being worked on in #4629. Once that's ready we should switch the VC to use the v3 endpoint.

A few other clients are also in the process of implementing the endpoint, so we'd need to come up with a sensible upgrade strategy.

• we probably want to ship this together with [Merged by Bors] - Block v3 endpoint #4629 and encourage user to update both Lighthouse BN and VC
• if user uses a different CL client that does not support V3 yet, do we introduce a flag to allow user to override the behaviour?
  • @michaelsproul suggested that we could use it by default once Deneb is enabled (all clients would have it ready by then), or have a flag to enable it prior to Deneb, so like:

if deneb_already_happened || config.use_v3 {
   beacon_node.get_v3(..)
} else {
   beacon_node.get_v2(..)
}

Beacon API spec:

• Add block v3 endpoint. ethereum/beacon-APIs#339

[eserilev]

i'd like to pick this one up

[jimmygchen]

Completed in #4813 🎉