Support PG18 virtual generated columns

Closes #3568

Author: roji

src/EFCore.PG/Migrations/NpgsqlMigrationsSqlGenerator.cs

@@ -1872,10 +1872,11 @@ protected override void ComputedColumnDefinition(
             throw new NotSupportedException("Computed/generated columns aren't supported in PostgreSQL prior to version 12");
         }
 
-        if (operation.IsStored != true)
+        if (operation.IsStored is not true && _postgresVersion < new Version(18, 0))
         {
             throw new NotSupportedException(
-                "Generated columns currently must be stored, specify 'stored: true' in "
+                "Virtual (non-stored) generated columns are only supported on PostgreSQL 18 and up. " +
+                "On older versions, specify 'stored: true' in "
                 + $"'{nameof(RelationalPropertyBuilderExtensions.HasComputedColumnSql)}' in your context's OnModelCreating.");
         }
 
@@ -1894,7 +1895,12 @@ protected override void ComputedColumnDefinition(
         builder
             .Append(" GENERATED ALWAYS AS (")
             .Append(operation.ComputedColumnSql!)
-            .Append(") STORED");
+            .Append(")");
+
+        if (operation.IsStored is true)
+        {
+            builder.Append(" STORED");
+        }
 
         if (!operation.IsNullable)
         {

src/EFCore.PG/Scaffolding/Internal/NpgsqlDatabaseModelFactory.cs

@@ -390,17 +390,22 @@ ORDER BY attnum
                     continue;
                 }
 
-                // Default values and PostgreSQL 12 generated columns
+                // Default values and generated columns
                 var defaultValueSql = record.GetValueOrDefault<string>("default");
-                if (record.GetFieldValue<string>("attgenerated") == "s")
+                switch (record.GetFieldValue<string>("attgenerated"))
                 {
-                    column.ComputedColumnSql = defaultValueSql;
-                    column.IsStored = true;
-                }
-                else
-                {
-                    column.DefaultValueSql = defaultValueSql;
-                    column.DefaultValue = ParseDefaultValueSql(systemTypeName, defaultValueSql);
+                    case "v":
+                        column.ComputedColumnSql = defaultValueSql;
+                        column.IsStored = false;
+                        break;
+                    case "s":
+                        column.ComputedColumnSql = defaultValueSql;
+                        column.IsStored = true;
+                        break;
+                    default:
+                        column.DefaultValueSql = defaultValueSql;
+                        column.DefaultValue = ParseDefaultValueSql(systemTypeName, defaultValueSql);
+                        break;
                 }
 
                 // Identify IDENTITY columns, as well as SERIAL ones.