Reference Intergration Plugin

To provide a plugin demonstrating pipeline integration that could be adapted at many studios.
For example, it shows users a shotgrid datasource plugin could be targeted.

The issue names no files, tests, or entry points. First clarify the plugin scope, the pipeline integration behavior, and whether the example should target a ShotGrid datasource; done should be a working reference plugin that studios can adapt.

Written by the indexing model from the issue text.