[sampleapp] Create Interface Actor

[srfc]

Issue

Build an example of how to generate an application interface to our top-level functions, e.g. using a HttpActor

Problem Description

• there is no bridge to any outside applications
• need to define some, e.g. http-, interface which provides synchronised access to our top-level functions

Supporting Information

• likely top-level functions needed for this interface:
  • start_session_as_customer(c_id) (is not implemented or defined yet)
  • add_items
  • checkout