Create a more rigid overseer builder pattern that fails at compile time (#4753)

Introduces `Missing<Field>` and `Init<Field>` states, that are used in place of builder generics, and make this possible.

polkadot/node/overseer/overseer-gen/src/tests.rs

@@ -7,3 +7,9 @@ fn ui_compile_fail() {
 	let t = trybuild::TestCases::new();
 	t.compile_fail("tests/ui/err-*.rs");
 }
+
+#[test]
+fn ui_pass() {
+	let t = trybuild::TestCases::new();
+	t.pass("tests/ui/ok-*.rs");
+}