Allow custom base class as node customization point (#3110)

Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>
Co-authored-by: Florian Albrechtskirchinger <falbrechtskirchinger@gmail.com>
Co-authored-by: barcode <barcode@example.com>

docs/examples/json_base_class_t.output

@@ -0,0 +1,4 @@
+/ - metadata = 42 -> {"null":null,"object":{"uint":1}}
+/null - metadata = 42 -> null
+/object - metadata = 21 -> {"uint":1}
+/object/uint - metadata = 42 -> 1