[C#] Support DTO generation via system property.

Previously, you had to pass the `CSharpDtos` FQN as the
`TargetCodeGenerator` when running sbe-tool; however, that means the XML
schema was parsed multiple times, as DTOs depend on flyweights,
which seems wasteful. Therefore, I have introduced a system property,
`sbe.csharp.generate.dtos` that also controls the generation of DTOs
when targetting `CSharp`.

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/csharp/CSharp.java

@@ -25,11 +25,28 @@
  */
 public class CSharp implements TargetCodeGenerator
 {
+    private static final boolean GENERATE_DTOS = Boolean.getBoolean("sbe.csharp.generate.dtos");
+
     /**
      * {@inheritDoc}
      */
     public CodeGenerator newInstance(final Ir ir, final String outputDir)
     {
-        return new CSharpGenerator(ir, new CSharpNamespaceOutputManager(outputDir, ir.applicableNamespace()));
+        final CSharpGenerator flyweightGenerator =
+            new CSharpGenerator(ir, new CSharpNamespaceOutputManager(outputDir, ir.applicableNamespace()));
+
+        if (GENERATE_DTOS)
+        {
+            final CSharpDtoGenerator dtoGenerator =
+                new CSharpDtoGenerator(ir, new CSharpNamespaceOutputManager(outputDir, ir.applicableNamespace()));
+
+            return () ->
+            {
+                flyweightGenerator.generate();
+                dtoGenerator.generate();
+            };
+        }
+
+        return flyweightGenerator;
     }
 }