Function encode_user_headers 

pub fn encode_user_headers(
    entries: &[(u8, &[u8], u8, &[u8])],
    buf: &mut BytesMut,
)

Encode header entries into a caller-owned buffer.

Each entry tuple is (key_kind, key_data, value_kind, value_data). The caller should pre-allocate buf with user_headers_encoded_size.