Function complete_proof_with_cert2 

async fn complete_proof_with_cert2<State>(
    state: &State,
    proof: &mut LeafProof,
    leaf: LeafQueryData<SeqTypes>,
    requested_leaf_qc: QuorumCertificate2<SeqTypes>,
    fetch_timeout: Duration,
    chain_limit: usize,
) -> Result<(), Error>
where
    State: AvailabilityDataSource<SeqTypes> + VersionedDataSource,
    for<'a> State::ReadOnly<'a>: NodeStorage<SeqTypes>,

Extend proof from leaf to the leaf directly committed by the earliest stored cert2, adding at most chain_limit leaves.