kuksa adapter should convert value from platform type (e.g. number) to string to fit to the VISS V2 standard

Currently, subscribed value from `kuksa.val` is not published to talents, if define the corresponding platform type is defined to number, because the type of responded value from `kuksa.val` is always string. This is also defined in the VISS v2 standard:

VISS V2 defined, that value to be set and read should be string. see https://raw.githack.com/w3c/automotive/gh-pages/spec/VISSv2_Transport.html#term-def

Attribute | Type | Description
-- | -- | --
value | string | The data value associated with the path.

Changing the type from number to string here works, but according to the `datatype` of vss model, it should be number (int32).
https://github.com/GENIVI/iot-event-analytics/blob/809993fa1a62b42d5afb9c95ec207f6094844d7c/src/sdk/javascript/examples/integrations/kuksa.val/config/platform/types.json#L19
Talked with @JochenKienzle, a type converter inside kuksa adapter will be implemented. The `datatype` of metadata should be then considered for convention.

Additional, `kuksa.val` accept now also non-string type for `setRequest`, which is not standard confirm. Therefore, the behavior will be changed in the future. An issue is already create for it: https://github.com/eclipse/kuksa.val/issues/209