Add support oneOf enum key

Now in generated classes with oneOf field require is using for checking not null only one element.
Proto:

  oneof location_oneof {
    ProtoBaseStationLocation BaseStationLocationInfo = 6;
    ProtoDeviceLocation DeviceLocationInfo = 7;
  }

Generated code in Wire:

  init {
    require(countNonNull(BaseStationLocationInfo, DeviceLocationInfo) <= 1) {
      "At most one of BaseStationLocationInfo, DeviceLocationInfo may be non-null"
    }
  }

But some servers send responses with oneOfField for choosing result as in protobuf lib. And wire failes decoding because countNotNull > 1.
Genereted code in ProtoBuf:

private int locationOneofCase_ = 0;
    private java.lang.Object locationOneof_;
    public enum LocationOneofCase
        implements com.google.protobuf.Internal.EnumLite {
      BASESTATIONLOCATIONINFO(6),
      DEVICELOCATIONINFO(7),
      LOCATIONONEOF_NOT_SET(0);

Start by locating Wire's oneof generation and decoding logic, using the shown generated Kotlin and protobuf Java representations as the comparison. Done means Wire can decode responses containing multiple oneof fields when servers use the protobuf-style oneof case representation, without failing the non-null check.