refactor: simplify filter plugin interface to use &mut dyn Read/Write

src/meta_plugin/exec.rs

@@ -127,7 +127,7 @@ impl MetaPluginExec {
             .stderr(Stdio::piped());
 
         match cmd.spawn() {
-            Ok(child) => {
+            Ok(mut child) => {
                 let stdin = child.stdin.take().unwrap();
                 self.writer = Some(Box::new(stdin));
                 self.process = Some(child);