feat(client): define constants and validation for DSCP and server fill

crates/irtt-client/src/config.rs

@@ -3,6 +3,8 @@ use std::{net::SocketAddr, time::Duration};
 use irtt_proto::{Clock, ReceivedStats, StampAt};
 
 pub(crate) const DEFAULT_PORT: u16 = 2112;
+pub const MAX_DSCP_CODEPOINT: u8 = 63;
+pub const MAX_SERVER_FILL_BYTES: usize = 32;
 pub(crate) const DEFAULT_DURATION: Duration = Duration::from_secs(3);
 pub(crate) const DEFAULT_INTERVAL: Duration = Duration::from_secs(1);
 pub(crate) const DEFAULT_OPEN_TIMEOUTS: [Duration; 4] = [