Make set_value a method within Serializer (encode#8001)

* Make set_value a static method for Serializers

As an alternative to encode#7671, let the method be overridden if needed. As
the function is only used for serializers, it has a better place in the
Serializer class.

* Set `set_value` as an object (non-static) method

* Add tests for set_value()

These tests follow the examples given in the method.

Author: tienne-B

tests/test_serializer.py

@@ -843,4 +843,4 @@ def test_exception_raised_when_data_and_instance_length_different(self):
                 instance=self.objs[:-1],
                 many=True,
                 partial=True,
-            )
+            )