Determine request telemetry Source based on SourceSystem

[stijnmoreels]

Is your feature request related to a problem? Please describe.
When we can track Azure Service Bus types #242 and we have SourceSystem in our request tracking model #244 , we should determine the Source of the request tracking telemetry.

Describe the solution you'd like
We should only provide a source when the SourceSystem is an Azure Service Bus type, otherwise the URL should be filled in with the request host and request URI.

We should make sure that this is future-proof, so we can add additional types later. This source system is better to be an enumeration or other strict type so we can control the different types.

Additional context
https://github.com/arcus-azure/arcus-service-to-service-correlation-poc

[stijnmoreels]

Done via #269