Add Charging Module generate bill run service

Once all the transactions have been added to the Charging Module bill run it needs to be [generated](https://defra.github.io/sroc-charging-module-api-docs/#/bill-run/GenerateBillRun). This is where it confirms the final values and calculates any additional transaction lines needed.

So, in this change we add app/services/charging-module/generate-bill-run.service.js which will handle calling the `/generate` endpoint in the [sroc-charging-module-api](https://github.com/DEFRA/sroc-charging-module-api).

Author: Cruikshanks