Allow users to register custom writers and readers for the procedural/scene format plugin

**Describe the feature you'd like**
Allow users to create and load custom writers and readers in the procedural and the scene format plugin.

Start by locating the procedural and scene format plugin entry points in the repository and trace how writers and readers are currently created and loaded. Clarify the registration lifecycle and supported interfaces with maintainers. Done means users can register custom writers and readers for both formats and load them through the plugins, with coverage for the supported registration paths.

Written by the indexing model from the issue text.