Flatten EIP-7685 requests, and requests_root

[somnathb1]

Rather than creating a trie and then using the trie root, use the hash of the rlp encoding of the requests list.

Ref1 - ethereum/EIPs#8854 ,
Ref2 - ethereum/EIPs#8907,
Ref3 - ethereum/EIPs#8924

(Tasks board - #12106)

[somnathb1]

While Ref1 here does say return keccak256(rlp.encode([rlp.encode(req) for req in list])) - I interpret the rlp encoding inside the loop as simply, the rlp encoding of the byte string that is req, and not the un-flattened rlp encoding we used have earlier.

[somnathb1]

Closing in favour of #12337