prove_message: RuntimeDescriptor<
    [leaf_index: bigint],

        | {
            leaf: FixedSizeBinary<32>;
            leaf_index: bigint;
            number_of_leaves: bigint;
            proof: FixedSizeBinary<32>[];
            root: FixedSizeBinary<32>;
        }
        | undefined,
>

Generate a merkle proof for a committed message identified by leaf_index. The merkle root is stored in the block header as a sp_runtime::generic::DigestItem::Other