feat(client): add Linux ancillary receive metadata

crates/irtt-client/Cargo.toml

@@ -7,8 +7,10 @@ rust-version.workspace = true
 
 [dependencies]
 irtt-proto = { path = "../irtt-proto" }
+libc = { version = "0.2", optional = true }
 socket2 = { version = "0.5", features = ["all"] }
 thiserror = "2"
 
 [features]
 default = []
+ancillary = ["dep:libc"]