feat: Add IFU CI support

[nickevansuk]

This PR adds IndividualFacilityUse functionality to the OpenActive Reference Implementation. IFUs have been the recommended approach to publishing facilities data for a while, so it is useful to have the ref impl aligned with this.

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In order to fully finish off this PR the following needs to happen (apart from the review comments below):

• 1. an appsettings.ifu.json file needs to be added to this PR (or based on the comment below re default FU vs IFU behaviour, an appsettings.fuNoIfu.json)
• 2. In Test Suite, an env file needs to be added to run Test Suite in IFU mode
• 3. This branch needs to be renamed to be coverage/ifus so that the Test Suite branch in 2 run tests against this branch
• 4. Add individual facility uses to the Github Actions test matrix so that every CI run also runs IFU tests