WrappedMessage.wrappedInstantNanos cannot be part of the scalarOrMessage oneof

Fields wrappedInstantSeconds and wrappedInstantNanos should be used together but it is illegal since it's a oneof. This definition is wrong and the error was not spotted in time because WrappedMessage is entirely handcoded.

To fix this,  one of the fields must become an optional field outside the oneof and keep the same field number to ensure compatibility.