DEPL-6248: rules example jetty-plugin

It seems to me that the work that is done here could be done completely in jython, thus proving the flexible extensibility of the rule system.

One approach would be 

1. Create a jython module that would do what this java class does. It can be called as `freemarker.xmlhelper`
1. Using planning script to define rule `jetty-dataSourceCreate` as follows 

```    
from freemarker import xmlhelper
helper = new XmlHelper
context.add(steps.template(freemarker_context={'helper': helper}, ....}
```

This way it's possible to enrich your freemarker with additional custom functionality without writing any java code.