Function prove_read 

pub fn prove_read<B, H, I>(
    backend: B,
    keys: I,
) -> Result<StorageProof, Box<dyn Error>>
where
    B: Backend<H>,
    H: Hasher,
    H::Out: Ord + Codec,
    I: IntoIterator,
    I::Item: AsRef<[u8]>,

Generate storage read proof.