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.

docs/src/js/interfaces/MaterializedViewDefinition.md

@@ -8,8 +8,8 @@
 
 The query that defines a materialized view, as stored:
 `SELECT columns FROM [ns.]table [, function(args) AS alias | , UNNEST(column) AS alias]
-[WHERE predicate] [LIMIT n]`. A Function in `FROM` position yields one row
-per element it returns.
+[WHERE predicate] [GROUP BY expr, ...] [LIMIT n]`. A Function in `FROM`
+position yields one row per element it returns.
 
 ## Properties