Prepare fluffy for Deneb/Cancun fork

[kdeme]

Adjust to any changes required here

[kdeme]

Looking at this, it seems nothing more than this PR is required: #2016
As long as we can properly decode the updated RLP BlockHeader, it should be good. (And this is supposed to be even compatible to addition of fields).

Looks like nothing should change to the SSZ BlockBody (nor Receipts) container for this fork. Blob data is ephemeral and not part of the BlockBody, so we should not store this in history network (It could go in a separate Portal network if wanted).

There is a new transaction type, but we already have transactions as RLP bytestrings in the SSZ container.
And even if we would need to decode them, we do support the new type in nim-eth.