Modify "Processing Time" of the shipping service

[feO2x]

We need to extend the ShippingService, written in Go, to allow setting the processing time.

• introduce JWT authentication and authorization to the service
• introduce a new endpoint where that lets the caller set a range from which a random processing time will be picked during shipping
• This new endpoint must only be called by authenticated users with the "admin" scope

This issue corresponds to Task 1 in #84.