ascanrules: Reflected XSS introduce param object

To facilitate further modifications and refactoring.

- CHANGELOG > Maint note already exists.
- CrossSiteScriptingScanRule > Introduce and leverage new param object.
- HtmlContextAnalyser > Throw an exception if the target param is empty
as that leads to an infinite loop.

Signed-off-by: kingthorin <[email protected]>

Author: kingthorin