Add support for generating metadata from runtime wasm files (#1720)

closes #1660

testing/ui-tests/src/correct/wasm_runtime_metadata.rs

@@ -0,0 +1,13 @@
+#[subxt::subxt(runtime_path = "../../../../artifacts/westend_runtime.wasm")]
+mod runtime {}
+
+#[subxt::subxt(runtime_path = "../../../../artifacts/westend_runtime.compact.compressed.wasm")]
+mod runtime_compressed {}
+
+fn main() {
+    use runtime;
+    use runtime_compressed;
+
+    let _ = runtime::system::events::CodeUpdated;
+    let _ = runtime_compressed::system::events::CodeUpdated;
+}