SSZ support for publishing blocks to the beacon node

[avalonche]

Is your feature request related to a problem? Please describe.
The new v2 publish block endpoint can accept either json or ssz. We should publish blocks to the beacon node using ssz with the v2 endpoint being supported in the relay. This will improve block publishing latency.

Describe the solution you'd like
Serialise the payload with ssz instead of json when publishing to the v2 endpoint.

[hopinheimer]

This will be a modification of original the PublishBlock() or do we keep the PublishBlock() as is for a fallback option. I see that the SubmitBlockRequestV2Optimitic is already implemented. I hope I'm making some sense in my findings 😅

[avalonche]

PublishBlock() will retain the functionality to switch between v1 and v2 but v2 will default to using ssz encoding.

[avalonche]

will bump this to high priority as may be causing too high latency:

"error":"write tcp write: broken pipe","level":"error","msg":"Couldn't write response"

on devnet 10.