Make header size functions public

[mullr]

I'm using this library in a streaming context, where I don't necessarily have the whole buffer available when reading data. By exposing these functions, I can interpret the framing protocol directly, allowing me to stream in a single message at a time.

For reference, my usage looks something like this:

/// Read a single, complete DLT message from `stream`, and parse it.
pub async fn read_dlt_message<S>(
    stream: &mut S,
) -> Result<dlt_core::parse::ParsedMessage, anyhow::Error>
where
    S: AsyncRead + Unpin,
{
    // dlt_core only works on a byte buffer. So here we interpret the
    // framing part of the protocol just enough to determine the
    // message size, then read it into a buffer, and pass that down to
    // dlt_core.

    // Read the first byte, from which we can calculate the header size
    let header_type_byte = stream.read_u8().await?;
    let headers_len = dlt_core::dlt::calculate_all_headers_length(header_type_byte) as usize;

    // Read the whole header
    let mut header_buf = vec![0u8; headers_len];
    header_buf[0] = header_type_byte;
    stream.read_exact(&mut header_buf[1..]).await?;
    let (_, header) = dlt_core::parse::dlt_standard_header(&header_buf)?;

    // Read the payload. Put it in a single buffer together with the header
    let total_message_size = headers_len + header.payload_length as usize;
    let mut msg_buf = vec![0u8; total_message_size];
    (&mut msg_buf[0..headers_len]).copy_from_slice(&header_buf);
    stream.read_exact(&mut msg_buf[headers_len..]).await?;

    let (remaining_data, dlt_msg) = dlt_core::parse::dlt_message(&msg_buf, None, false)?;
    if !remaining_data.is_empty() {
        return Err(anyhow!(
            "Remaining data after loading DLT message: {remaining_data:?}"
        ));
    }

    Ok(dlt_msg)
}