C#: Introduce extractor mode to identify DBs created with codeql test run

csharp/extractor/Semmle.Extraction/Extractor/ExtractorMode.cs

@@ -0,0 +1,16 @@
+using System;
+
+namespace Semmle.Extraction
+{
+    /// <summary>
+    /// The mode in which a file is extracted.
+    /// </summary>
+    [Flags]
+    public enum ExtractorMode
+    {
+        None = 0,
+        Standalone = 1,
+        Pdb = 2,
+        QlTest = 4,
+    }
+}