Extend PoolTransaction trait with tx_type and length

[mattsse]

Describe the feature

with eth68 newPooledTransaction Hashes (#1482) we also need to send the type and encoded length of the transaction.

This can already be obtained by converting PoolTransaction -> TransactionSigned and then get the length by encoding the transaction.
however, this is pretty wasteful because we need to reencode the transaction everytime it is requested or announced.

TODO

• extend PoolTransaction trait with two more functions:
  • fn tx_type() -> u8 returns the type of the transaction
  • fn encoded_length() -> usize returns the length of the rlp encoded object. This is then cached in the ValidPoolTransaction type as a field.

Additional context

No response

[leruaa]

Is this blocked by #1482? If not I would like to take this on.

[mattsse]

unblocked now