Persistence JDBC components (for compile-time injection).
The serializer registry.
If no JSON serializers need to be provided, this can simply return EmptyJsonSerializerRegistry.
The optionally provided serializer registry.
Note that this can also be exploited to allow multiple traits to contribute to the serializer registry rather
than provide one, by checking whether the super implementation also provides one, and concatenating with that if
it does. To do that, the override must be a def, so that it can be subsequently overridden by other mixed in