Support scalapb oneof sealed_value

[johanandren]

ScalaPB has a special quirky feature which is that a protobuf oneof with the name sealed_[name] will generate a sealed ADT (docs: https://scalapb.github.io/docs/sealed-oneofs/) however that generates types that the SDK serialization logic AnySupport doesn't expect.

Doesn't seem very complicated to support, the core of the problem is that the GeneratedMessageCompanion cannot be found the usual way for such generated sealed Scala ADTs.