feat(client): implement DSCP negotiation and socket traffic class application

Apply the negotiated DSCP value to the UDP socket after a successful session open and clear it before closing to support network-level quality of service. This change adds platform-specific socket option handling for IPv4 TOS and IPv6 Traffic Class.

crates/irtt-client/Cargo.toml

@@ -7,7 +7,7 @@ rust-version.workspace = true
 
 [dependencies]
 irtt-proto = { path = "../irtt-proto" }
-socket2 = "0.5"
+socket2 = { version = "0.5", features = ["all"] }
 thiserror = "2"
 
 [features]