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