feat: update VirtualTable to use an expression for defining table records

proto/substrait/algebra.proto

@@ -147,10 +147,29 @@ message ReadRel {
     }
   }
 
+  // Wrapper for repeated literal struct records
+  message LiteralRecords {
+    repeated Expression.Literal.Struct values = 1;
+  }
+
+  // Wrapper for repeated nested struct expressions
+  message NestedRecords {
+    repeated Expression.Nested.Struct expressions = 1;
+  }
+
   // A table composed of expressions.
   message VirtualTable {
-    repeated Expression.Literal.Struct values = 1 [deprecated = true];
-    repeated Expression.Nested.Struct expressions = 2;
+    oneof records_type {
+      LiteralRecords values = 1 [deprecated = true];
+      // Represents table data as a list of nested struct expressions.
+      // Each struct corresponds to a record, with expressions defining column values.
+      NestedRecords expressions = 2;
+      // Represents table data as an expression that should evaluate to a list of structs,
+      // i.e., its type is LIST<STRUCT<T1,...,Tn>>.
+      // This approach provides flexibility, allowing the use of a dynamic parameter
+      // to define the virtual table's content.
+      Expression record_list_expression = 3;
+    }
   }
 
   // A stub type that can be used to extend/introduce new table types outside

site/docs/relations/logical_relations.md

@@ -50,15 +50,20 @@ Read definition types (like the rest of the features in Substrait) are built by
 
 #### Virtual Table
 
-A virtual table is a table whose contents are embedded in the plan itself.  The table data
-is encoded as records consisting of literal values or expressions that can be resolved without referencing any input data.
-For example, a literal, a function call involving literals, or any other expression that does
-not require input.
+A virtual table is a table whose contents are embedded within the plan itself.
+The table data can be represented in one of two ways:
 
-| Property | Description | Required |
-| -------- | ----------- | -------- |
-| Data     | Required    | Required |
+* As explicit records - a list of struct-based expressions defining each record directly.
+* As a single expression - an expression that evaluates into a list of records, with the type `LIST<STRUCT<T1,...,Tn>>`.
 
+The records should consist of literal values or expressions that can be resolved without referencing any input data.
+For example, a literal, a function call involving literals, or any other expression that does not require input.
+Encoding the data as an expression using nested types provides flexibility, allowing the use of a dynamic parameter
+expression as a single expression for the virtual table.
+
+| Property | Description       | Required |
+| ---------| ----------------- | -------- |
+| Data     | A list of records | Required |
 
 #### Named Table