Handle datatypes for allowed values

VSS allows the specification of any datatype for allowed values.
```yaml
FuelType:
type: attribute
allowed: ["DIESEL","E10","E5"]
datatype: string
...

Gear:
type: sensor
allowed: [1,2,3,4,5]
datatype: int
```

GraphQL, however represents values of enums as String:
```
enum GearEnum {
_1
_2
_3
}
```
The revolvers can map the enum to any datatype.

--
How to handle that?