OPC-UA Client: Support for ExtensionObjects

## Feature Request
In more complex OPC-UA server architectures there is heavy use of extension objects. An ExtensionObject is a container for any Structured DataTypes which cannot be encoded as one of the other built-in data types. A prime example for this would be an alert:
```
struct custom_alert {
bool door_open;
int32_t current_operating_mode;
float temperature;
};
```

### Proposal:
We should provide functionality within the OPC-UA client for deserializing Extention Objects. This could be done by pre-declaring Extention Objects to the client. I believe gopcua does support an Extention Object repository though I do not know if these objects need to be declared before the binary is built.