Use Exception.ToString in headers

Two modes

### Default

Replace the value written to `NServiceBus.ExceptionInfo.StackTrace` with `Exception.ToString`

### Legacy

Force exception headers back to `legacy mode` ie keep original headers and use `Exception.StackTrace` in `NServiceBus.ExceptionInfo.StackTrace`.

Enabled by setting the `NServiceBus/Headers/UseLegacyExceptionStackTrace` AppSetting to `true`.