fix: add postgres backend

changes:
- Add postgres backend
- Modified or added table attributes.
  UpdateDate, ForeignKeys, Booleans
- Fix test for sqlite and postgres. Compare json instead the struct.

pkg/types/measuredValue.go

@@ -13,6 +13,6 @@ type MeasuredValue struct {
 	ValueType    string     `json:"value_type" xml:"value_type"`
 	Date         time.Time  `json:"date" xml:"date"`
 	SensorID     string     `json:"sensor_id" xml:"sensor_id"`
-	CreationDate *time.Time `json:"creation_date" xml:"creation_date"`
+	CreationDate time.Time  `json:"creation_date" xml:"creation_date"`
 	UpdateDate   *time.Time `json:"update_date" xml:"update_date"`
 }