Add new method(s) in serializer/deserializer factory for accessing only "POJO" (de)serializer

Although lookups by general Java type (Collection(-like), Map(-like), array, Enum, reference type, "other") works well for registering custom (de)serializers, there is one set of functionality where it is not sufficient: that of "shape-changing".

For example, when serializing `Enum` values "as POJOs" (by inspecting properties), lookup is needed ONLY for POJO serializer (`BeanSerializer`), and not for Enum type. But currently there is no way to explicitly specify desire for such (de)serializer. This requires clumsy work-arounds on factories, lack of extensibility. With 3.x we can add new methods to overcome this problem.