Move reusable supplementary billing services

https://eaflood.atlassian.net/browse/WATER-4201

As we write this description the long outstanding annual SROC two-part tariff bill runs are being 'sent' in production.

The final part of two-part tariff is to add support for supplementary billing; when changes are made to a licence or the returns previously submitted and billed, we need to generate supplementary two-part tariff bill runs.

We've already been making a number of changes in support of it, off the back of a [spike](#1412).

That spike has shown us there are existing billing services, especially in supplementary billing, that we can reuse for two-part supplementary. This change moves them to make their reuse clearer, whilst tweaking a few things to allow the reuse to happen.