+// `$decode_custom_tlv` is a closure that may be optionally provided to handle custom message types.
+// If it is provided, it will be called with the custom type and the `FixedLengthReader` containing
+// the message contents. It should return `Ok(true)` if the custom message is successfully parsed,
+// `Ok(false)` if the message type is unknown, and `Err(DecodeError)` if parsing fails.