Function write_raw_response 

pub fn write_raw_response(
    w: &mut impl Write,
    opcode_byte: u8,
    meta: &ResponseMeta,
    extras: &[u8],
    key: &[u8],
    value: &[u8],
) -> Result<()>

Write a complete binary-protocol response to w using a raw opcode byte. This is the low-level writer; prefer write_response when you have a known Opcode.