[C++] Generate DTOs for non-perf-sensitive usecases.

In some applications performance is not cricital. Some users would like
to use SBE across their whole "estate", but don't want the "sharp edges"
associated with using flyweight codecs, e.g., accidental escape.

In this commit, I've added a first cut of DTO generation for C++ and a
simple test based on the Car Example.

The DTOs support encoding and decoding via the generated codecs using
`DtoT::encode(CodecT& codec, const DtoT& dto)` and
`DtoT::decode(CodecT& codec, Dto& dto)` methods.

Generation can be enabled specifying the target code generator class,
`uk.co.real_logic.sbe.generation.cpp.CppDtos`, or by passing a
system property `-Dsbe.cpp.generate.dtos=true`.

sbe-tool/src/main/java/uk/co/real_logic/sbe/SbeTool.java

@@ -192,6 +192,11 @@ public class SbeTool
      */
     public static final String DECODE_UNKNOWN_ENUM_VALUES = "sbe.decode.unknown.enum.values";
 
+    /**
+     * Should generate C++ DTOs. Defaults to false.
+     */
+    public static final String GENERATE_CPP_DTOS = "sbe.cpp.generate.dtos";
+
     /**
      * Configuration option used to manage sinceVersion based transformations. When set, parsed schemas will be
      * transformed to discard messages and types higher than the specified version. This can be useful when needing

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

@@ -15,6 +15,7 @@
  */
 package uk.co.real_logic.sbe.generation;
 
+import uk.co.real_logic.sbe.generation.cpp.CppDtoGenerator;
 import uk.co.real_logic.sbe.generation.java.JavaOutputManager;
 import uk.co.real_logic.sbe.generation.c.CGenerator;
 import uk.co.real_logic.sbe.generation.c.COutputManager;
@@ -81,10 +82,20 @@ public CodeGenerator newInstance(final Ir ir, final String outputDir)
          */
         public CodeGenerator newInstance(final Ir ir, final String outputDir)
         {
-            return new CppGenerator(
-                ir,
-                "true".equals(System.getProperty(DECODE_UNKNOWN_ENUM_VALUES)),
-                new NamespaceOutputManager(outputDir, ir.applicableNamespace()));
+            final NamespaceOutputManager outputManager = new NamespaceOutputManager(
+                outputDir, ir.applicableNamespace());
+            final boolean decodeUnknownEnumValues = "true".equals(System.getProperty(DECODE_UNKNOWN_ENUM_VALUES));
+
+            final CodeGenerator codecGenerator = new CppGenerator(ir, decodeUnknownEnumValues, outputManager);
+            final CodeGenerator dtoGenerator = new CppDtoGenerator(ir, outputManager);
+            final CodeGenerator combinedGenerator = () ->
+            {
+                codecGenerator.generate();
+                dtoGenerator.generate();
+            };
+
+            final boolean generateDtos = "true".equals(System.getProperty(GENERATE_CPP_DTOS));
+            return generateDtos ? combinedGenerator : codecGenerator;
         }
     },