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

crates/irtt-app/src/shared/hmac.rs

@@ -0,0 +1,14 @@
+/// Decode CLI key syntax without ever including key material in diagnostics.
+pub(crate) fn decode_hmac_key(input: &str) -> Result<Vec<u8>, String> {
+    let key = match input.strip_prefix("0x") {
+        Some(hex) => hex::decode(hex).map_err(|_| {
+            "HMAC hexadecimal key must contain an even number of valid hexadecimal digits"
+                .to_owned()
+        })?,
+        None => input.as_bytes().to_vec(),
+    };
+    if key.is_empty() {
+        return Err("HMAC key must not be empty".to_owned());
+    }
+    Ok(key)
+}