create serde_deprecated feature flag

Author: AHaliq

Cargo.toml

@@ -52,3 +52,7 @@ tokio-test = { version = "0.4" }
 tonic = {version = "0.12.0", features = ["tls", "tls-roots"]} # Use system trust roots.
 tracing-subscriber = "0.3"
 sqlite = "0.33"
+
+[features]
+default = ["serde_deprecated"]
+serde_deprecated = [] # enables `serde` for composite data structures; see `README.md` for more information

README.md

@@ -103,3 +103,7 @@ cargo run
 
 Updating these files should only be done when the node's API, determined by the
 schemas, changes and we need to support the new API in the SDK.
+
+The flag `serde_deprecated` guards `serde::Serialize` and `serde::Deserialize`
+implementations for composite data structures where these implementations will
+eventually be deprecated. Enable the flag to use them.