feat: group a materialized view by GROUP BY

A view could only map source rows one to one, or one to many through a
FROM-position item, so nothing could see all the rows sharing a key.

A view query now takes `GROUP BY expr, ...` with aggregate projections,
planned and executed by DataFusion over the lance scan. A group spans
fragments, so a grouped view is recomputed in full whenever its source
changes; each row's provenance is its group's smallest source row id.
Such a query is stored as definition format 2, so a reader that predates
grouping reports the view as unrefreshable instead of failing to parse it.

nodejs/__test__/materialized_view.test.ts

@@ -54,7 +54,7 @@ describe("materialized views", () => {
 
     // A newer writer's layout is reported, never guessed at.
     for (const newer of [
-      `{"format":2,"query":${JSON.stringify(query)}}`,
+      `{"format":3,"query":${JSON.stringify(query)}}`,
       '{"kind":"select_v3","source_table":"people"}',
     ]) {
       expect(() => read(newer)).toThrow(/cannot refresh/);