CrossCutting: abstract IJsonSerializer

[aahmed-dfe]

Currently we call to different JSON serialiser impl (NewtonSoft, Microsoft.Text..) through static methods.

We should invert this through IOC e.g. IJsonSerialiser so there is consistent usage of a serialiser, which we can extend in the future through options / patterns when unique usecases appear.

There is an existing simple impl in MyPupils where a basic NewtonsoftJsonSerialiser has been written. Register this as a singleton and start applying.

Note v1.8.1 had an issue where establishment users could not login as Auth didn't deserialise (no users)

[aahmed-dfe]

Simple inversion started... start moving everything to consume abstraction....

[aahmed-dfe]

TODO with testing would we be better off, resolving IJsonSerialiser via the composition root, and asserting against the contract ... should we ever swap impl.