Add rules function for converting data element values to string

As a Mobile SDK user configuring rules remotely in Launch UI, I want to be able to fetch any node in a shared state or event data, regardless its type, and be able to convert that to string when the rules are evaluated.
I want to have a function in rules that allows me to do that when the a condition or action is evaluated.

For example, I should be able to send a post back including the visitor identifiers:
**{%toString(~state.com.adobe.module.identity/visitoridslist)%}**
**{%toString(~xdmstate.com.adobe.module.identity/identityMap)%}**

note: `~state.com.adobe.module.identity/visitoridslist` is a free form data element

Start by locating the rules condition and action evaluation paths and how free-form data elements resolve shared-state and event-data nodes. Trace the existing value handling, then verify that the proposed toString function accepts values of any type and produces the requested string output for the example state paths.

Written by the indexing model from the issue text.