Support for configs polymorphism

[andrus]

Jackson supports polymorphic type resolution for configuration based on a value of a specific property in YAML (e.g. type: mytype). A supertype (or interface) is annotated with @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "xyz") and concrete subclasses - with @JsonTypeName("abc"). This is great for dynamic configurations where each possible subclass has different set of properties. (E.g. FileAppender vs. ConsoleAppender).

Current workaround is to use Map<String, String> and manually binding properties.

[andrus]

Added unit tests. The feature itself is built into Jackson, so not assigning this to any particular release.

[andrus]

Reopening... Enumerating subclasses with @JsonSubTypes annotation is completely inflexible. Dropwizard has a rather ingenious approach - combining Jackson with ServiceLoader mechanism and a tag interface for all polymorphic factory classes. We should use something similar.