refactor+feat: allow subsystems to send only declared messages, generate graphviz (#5314)

Closes #3774
Closes #3826

polkadot/node/overseer/overseer-gen/proc-macro/Cargo.toml

@@ -17,12 +17,19 @@ quote = "1.0.18"
 proc-macro2 = "1.0.37"
 proc-macro-crate = "1.1.3"
 expander = { version = "0.0.6", default-features = false }
+petgraph = "0.6.0"
 
 [dev-dependencies]
 assert_matches = "1.5.0"
+polkadot-overseer-gen = { path = "../" }
+thiserror = "1"
+gum = { package = "tracing-gum", path = "../../../gum" }
 
 [features]
-default = []
+default = ["graph", "expand"]
 # write the expanded version to a `overlord-expansion.[a-f0-9]{10}.rs`
 # in the `OUT_DIR` as defined by `cargo` for the `expander` crate.
 expand = []
+# Create directional message consuming / outgoing graph.
+# Generates: `${OUT_DIR}/${overseer|lowercase}-subsystem-messaging.dot`
+graph = []