Function set_header_footer 

pub fn set_header_footer(
    ws: &mut WorksheetXml,
    header: Option<&str>,
    footer: Option<&str>,
) -> Result<()>

Set header and footer text for printing.

Both parameters are optional. Pass None to leave a field unchanged. Excel header/footer text uses special formatting codes (e.g., &L, &C, &R for left/center/right sections).