Custom Node Group As Modifier

[samdutter]

Pre-Flight checklist

• [x] Did you check to see if this issue already exists?
• [x] This is a single feature request. (Do not put multiple feature requests in one issue)

Describe The Feature Request Below

In addition to the current modifiers that exist, adding in the ability to select a shader group node as a modifier would be amazing. Could easily add blurring, and other interesting techniques into the current pipeline while making it very customizable/flexible.

[ucupumar]

This is similar to #90, but since it's specifically for the modifiers, it's probably not a duplicate.