Support for large payloads.

[michaelstaib]

The current generator will only accept value types and string as payloads to be compliant with etw. There are cases were customers still want to generate an event source with custom objects for which they handle the payload serialisation in an nonevent method.

The template info file should define which types are accepted by a template. Moreover, a template should explicitly in- or exclude types or work with wildcards.

[michaelstaib]

I think for the first version we will have just a simple bool switch in the template settings file "allowLargePayloads". The switch default is false in order to not affect existing templates.

If large payloads are disabled we will throw an exception like we do now. If large payloads are enabled the generator can use new metadata provided to the generator model to generate special code that handles large payloads.

We should add another cc template that shows how large payload handling could be implemented.