EIP-4844 blob sidecars are a concept that only exists in the mempool.
After inclusion of a transaction into an execution block, only the
versioned hash within the transaction remains. To improve type safety,
replace the `Transaction.networkPayload` member with a wrapper type
`PooledTransaction` that is used in contexts where blob sidecars exist.
This is more according to the structure of the code itself.
And also, currently some of these tests live under tests/rlp but
some under tests/common. Could use some further re-organisation
within the different tests, but this move is a first step.