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

[cowtowncoder]

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.