feat: add VirtualExpressionTable definition and use it in ReadRel

This enables expressing Virtual table having values as expression

proto/substrait/algebra.proto

@@ -73,10 +73,11 @@ message ReadRel {
 
   // Definition of which type of scan operation is to be performed
   oneof read_type {
-    VirtualTable virtual_table = 5;
+    VirtualTable virtual_table = 5 [deprecated = true];
     LocalFiles local_files = 6;
     NamedTable named_table = 7;
     ExtensionTable extension_table = 8;
+    VirtualExpressionTable virtual_expression_table = 9;
   }
 
   // A base table. The list of string is used to represent namespacing (e.g., mydb.mytable).
@@ -91,6 +92,11 @@ message ReadRel {
     repeated Expression.Literal.Struct values = 1;
   }
 
+  // A table composed of expressions.
+  message VirtualExpressionTable {
+    repeated Expression values = 1;
+  }
+
   // A stub type that can be used to extend/introduce new table types outside
   // the specification.
   message ExtensionTable {

site/docs/relations/logical_relations.md

@@ -57,6 +57,16 @@ is encoded as records consisting of literal values.
 | -------- | ----------- | -------- |
 | Data     | Required    | Required |
 
+#### Virtual Expression Table
+
+A virtual expression table is a table whose contents are embedded in the plan itself.  The table data
+is encoded as records consisting of expression values.
+
+| Property | Description | Required |
+| -------- | ----------- | -------- |
+| Data     | Required    | Required |
+
+
 #### Named Table
 
 A named table is a reference to data defined elsewhere.  For example, there may be a catalog