feat: support aggregate in scanner

rust/lance-datafusion/src/aggregate.rs

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The Lance Authors
+
+//! Aggregate specification for DataFusion aggregates.
+
+use datafusion::logical_expr::Expr;
+
+/// Aggregate specification with group by and aggregate expressions.
+#[derive(Debug, Clone)]
+pub struct Aggregate {
+    /// Expressions to group by (e.g., column references).
+    pub group_by: Vec<Expr>,
+    /// Aggregate function expressions (e.g., SUM, COUNT, AVG).
+    /// Use `.alias()` on the expression to set output column names.
+    pub aggregates: Vec<Expr>,
+}

rust/lance-datafusion/src/lib.rs

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: Apache-2.0
 // SPDX-FileCopyrightText: Copyright The Lance Authors
 
+pub mod aggregate;
 pub mod chunker;
 pub mod dataframe;
 pub mod datagen;