Use v3 beacon API to get block

[y0sher]

Description

Currently there are two different beacon routes to get blocks, under v2 for unblinded blocks - [/eth/v2/validator/blocks/{slot}](https://ethereum.github.io/beacon-APIs/#/Validator/produceBlockV2)
and under v1 for blinded blocks - /eth/v1/validator/blinded_blocks/{slot}.

This results in code where we have to request from different API endpoints and do different handling based on the builderProposal settings in the SSV node, also blinded blocks might be turned off in the beacon node and the route won't work, even more, the MEV relay/boost might still fail to create a blinded block and we'll need to request again a non-blinded block as a fallback.

Solution

/eth/v3/validator/blocks/{slot} was introduce to return a valid blinded or non-blinded block in all cases, without need for special handling. change our proposals code to use this route.