INT-3130: Add result-type to <o-to-j-transformer>

JIRA: https://jira.springsource.org/browse/INT-3130

* add `result-type` attribute to the `<object-to-json-transformer>`
based on enumeration values: `STRING`, `NODE`
* Provide implementations for methods to convert value to the node representation
* Refactoring according new generic option in the `JsonObjectMapper`
* Add tests and documentation

Need Migration Guide note according this refactoring