Test Execution Endpoint

[raphael-goetz]

My thoughts:

• A new Endpoint, same functionality as FlowService, Aquila sends a logon request. Sagittarius sends an execution request and Aquila responds with the result. Both are a stream.
• Not inside the FlowService, I thought the FlowService is already full and should only handle flows, not executions. If introducing a new separate stream is to resource intensive we just could merge it into the FlowService, the same argumentation will apply on the ActionService
• Optionally, if a Sagittarius receives a logon request via the FlowService, it should have the address and could connect back to Aquila (?) Then the ExecutionService could be just a server endpoint of Aquila

(Also the requests aren't needed in shared as only Sagittarius and Aquila will ever handle the test execution, the concept is unknown to Draco and Taurus)

Resolves: #41