[FEATURE REQUEST] pillar:// as a way of ambiguously target pillar files regardless of filesystem.

Is your feature request related to a problem? Please describe.
I know this won't be an easy feature to create so just going to start out with the basic's sometimes it is difficult to do jinja or python from within pillar cause as soon as you need to access another file you need to know how the file is loaded from what file system. if we had a way of accessing the file regardless of if we are using gitfs or other methods. that disambiguates the pillar filesystem for tools like slsutil.renderer.

Describe the solution you'd like
a new keyword kind of like salt:// that says this is in the pillar filesystem. such as pillar:// it should only be accessible from within pillar templates.

Describe alternatives you've considered
using salt:// to "smartly" decide where the file is. might be a better option but can be confusing to someone coming in without a clear idea that salt:// normally means the salt fileserver.

Start by tracing how slsutil.renderer resolves salt:// references from pillar templates and how those references select a filesystem. Define what pillar:// should resolve to across supported filesystems and restrict it to pillar templates; done means the behavior is implemented and verified for the requested use case.