Add the [MessagePackFormatter] attribute when MessagePack conversion is enabled

[aradalvand]

Describe the feature

MessagePack support was added in #701, now a MessagePack formatter is generated for every value object, which is great. But the formatters have to be manually registered right now since the value object isn't annotated with the [MessagePackFormatter] attribute (analogous to System.Text.Json's [JsonConverter] attribute), which makes it so that the formatters are automatically picked up by the MessagePack serializer.

Please consider simply adding a [MessagePackFormatter(typeof(<FormatterType>))] on top of the generated value objects when MessagePack conversion is enabled, like some of the other converters.

[SteveDunn]

Thanks for the feedback! Released in https://github.com/SteveDunn/Vogen/releases/tag/5.0.6-beta.1