actually add filter skeleton

eggstrain/src/execution/operators/filter.rs

@@ -0,0 +1,34 @@
+use super::{Operator, UnaryOperator};
+use arrow::record_batch::RecordBatch;
+use async_trait::async_trait;
+use datafusion::physical_plan::ExecutionPlan;
+use std::sync::Arc;
+use tokio::sync::broadcast;
+
+pub struct Filter {
+    pub children: Vec<Arc<dyn ExecutionPlan>>,
+}
+
+impl Operator for Filter {
+    fn children(&self) -> Vec<Arc<dyn ExecutionPlan>> {
+        self.children.clone()
+    }
+}
+
+#[async_trait]
+impl UnaryOperator for Filter {
+    type In = RecordBatch;
+    type Out = RecordBatch;
+
+    fn into_unary(self) -> Arc<dyn UnaryOperator<In = Self::In, Out = Self::Out>> {
+        Arc::new(self)
+    }
+
+    async fn execute(
+        &self,
+        mut rx: broadcast::Receiver<Self::In>,
+        tx: broadcast::Sender<Self::Out>,
+    ) {
+        todo!()
+    }
+}