use gorm serialiser instead of custom hooks

[kradalby]

This pr removes the custom types we have to wrap things to put them in the database, instead, we use gorm serialisers: https://gorm.io/docs/serializer.html.

We use the standard JSON serialiser and we introduce a Text serialiser that allows us to serialise types that implements: https://pkg.go.dev/encoding#TextMarshaler.

This allows us to remove even more custom code handling and a bunch of type casting/conversion.