feat(app): allow hexadecimal HMAC key specification and redact keys from debug output

crates/irtt-app/Cargo.toml

@@ -52,6 +52,7 @@ clap = { version = "4.6.7", features = ["derive", "env"] }
 clap-num = "=1.2.0"
 crossterm = { version = "0.29.0", features = ["event-stream"], optional = true }
 futures-core = { version = "0.3.34", default-features = false, optional = true }
+hex = "0.4.3"
 ctrlc = { version = "3.5.2", features = ["termination"] }
 irtt-client = { workspace = true, optional = true }
 irtt-proto = { workspace = true, optional = true }