feat: add proc macro to reduce overseer mock boilerplate (#2949)

polkadot/node/overseer/subsystems-gen/tests/ui/err-01-no-generic.rs

@@ -0,0 +1,17 @@
+#![allow(dead_code)]
+
+use polkadot_procmacro_overseer_subsystems_gen::AllSubsystemsGen;
+
+#[derive(Clone, AllSubsystemsGen)]
+struct AllSubsystems {
+	a: f32,
+	b: u16,
+}
+
+fn main() {
+	let all = AllSubsystems {
+		a: 0_f32,
+		b: 1_u16,
+	};
+	let _all = all.replace_a(77u8);
+}