Support scope variables in URIs

[timhendriks93]

These changes allow the usage of scope variables within URIs. This can be helpful to create scenarios in which you would want to request URLs with dynamic segments (e.g. resource IDs). To avoid repetition, I created a new helper function to build Request URLs by concatenating the URI and the base URL while replacing scope variables.

Example scenario flow

1. A POST request creates a new resource and responds with the new resource ID (e.g. 123)
2. The resource ID is extracted from the response and saved into a scope variable (e.g. "id")
3. A GET request to the newly created resource is made (e.g. /resource/##id)
4. The scenario verifies that the status code equals 200 (not 404)

[JoramM]

Looks good.