Add unit type to the DTO

[haimkastner]

To make the schema and documentation clear, and to support full units DTO, including the type itself (Length, Angle, etc).
Like this:

{
   "value":100.01,
   "quantity":"Meter",
   "unit": "Length"
}

Implement it while still keeping BC to the current DTO implementation.

See discussion on angularsen/UnitsNet#1378