Decode signed blinded beacon block with consensus version header

In the latest builder spec (0.3.0), [the `submitBlindedBlock` endpoint](https://github.com/ethereum/builder-specs/blob/main/apis/builder/blinded_blocks.yaml#L19) adds an additional (optional) `Eth-Consensus-Version` request header, which has the fork name as value. This was added so that the receiver could use this information to deseralize the request body easily.

`mev-boost` currently tries to decode using Capella, and then falls back to Bellatrix if it fails. The newly added version header could be utilised here to avoid the decode attempts and potentially catch errors like invalid payload for the fork.

https://github.com/flashbots/mev-boost/blob/main/server/service.go#L697-L709