added serde as optional dependecy

I mean for real: How patient do you'll need to be to not get annoyed by the whole serde
warnings?

Author: GueLaKais

rclrs/Cargo.toml

@@ -27,6 +27,7 @@ libloading = { version = "0.8", optional = true }
 
 # Needed for the Message trait, among others
 rosidl_runtime_rs = "0.4"
+serde = { version = "1.0.210", optional = true }
 
 [dev-dependencies]
 # Needed for e.g. writing yaml files in tests