beacon/engine,eth/catalyst: hex marshal requests in engine api

[lightclient]

In #30584 I was mostly trying to sort out the issues in t8n, now I've run into similar unmarshaling issues in the engine API. Currently the methods like NewPayloadV4 define requests as [][]byte, but this doesn't handle hex correctly.

The two solutions to this seems to be:

• define a type like RequestsList with the decoding methods
• create a list of hexutil.Bytes

I went with the second option in this PR. It seems overkill to create a fully new type. However, what gives me pause is the interface for BlockToExecutableData now takes in []hexutil.Bytes. We could keep it as [][]byte, but that would require some data shuffling to convert from the []hexutil.Bytes. Not sure if people have a preference on that parameter type.

[karalabe]

I'd like to push back against hexutil types being passed around as method params. The purpose of those is to be encoding stuff. I'd ideally prefer to convert from hexutil to []byte and pass that to the block makers. Otherwise you open up a can of worms where hexutil will start to worm itself into our internal typesystem.

[fjl]

I have pushed another small update. Not sure if using unsafe is really warranted, but it feels silly to allocate a new slice just to convert to the same underlying type.

[fjl]

Actually changed my mind, it's better without unsafe.

[lightclient]

I feel like if we're going to have requests as [][]byte in ExecutionPayloadEnvelope we should also update the witness to be []byte?