feat: enhance VirtualTable to have expression as value (#711)

* This enables expressing Virtual table having values as expression
* Example query: `select * from (values (1+2, 'Hello'||'World'))`
* Mark existing literal field "values" as deprecated

proto/substrait/algebra.proto

@@ -86,9 +86,10 @@ message ReadRel {
     substrait.extensions.AdvancedExtension advanced_extension = 10;
   }
 
-  // A table composed of literals.
+  // A table composed of expressions.
   message VirtualTable {
-    repeated Expression.Literal.Struct values = 1;
+    repeated Expression.Literal.Struct values = 1 [deprecated = true];
+    repeated Expression expressions = 2;
   }
 
   // A stub type that can be used to extend/introduce new table types outside

site/docs/relations/logical_relations.md

@@ -51,12 +51,15 @@ 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.
+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.
 
 | 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