Add ParameterStorageService to handle sensitive parameters which can not be stored in plaintext.

Some component parameters are sensitive and can not be stored in the KnowledgeBase (for document sources) or in plain configuration files (like the workbench) or XWiki objects.

We need a storage service for these parameters with an interface similar to:

interface ParameterStorage {
  Object getParameter(String componentName, String parameterName);
  void setParameter(String componentName, String parameterName, Object parameterValue);
}

To call it, the client should just have to do:

Scribo.getParameterStorageService().getParameter("myDS", "password")

Specifying the implementation could be done trough a JVM parameter like:
-DscriboParameterStorage=some.package.KDEWalletStorage

Other configuration of this service (like secret key, path, etc.) can be done trough JVM parameters as well and the storage service will be permanently accessible. This way, the parameters are accessible inside the JVM, but not from the outside (filesystem or other JVMs).