IO Extension to support exporter

[hoshiryu]

Currently RadiumIO only deals with loaders, and the namespace Core::Asset only defines the interface for loaders, except for the deprecated directory. This prevents to export data to file formats other than OBJ, MAP and OFF, whose exporters are too shallow, without having to manually write the exporter, without any defined connection to the RadiumEngine.

We thus need to extend the Ra::Asset namespace so that it also handles exporters. I suggest renaming the FileLoaderInterface into FileManagerInterface and add a save method to it (inspired by the FileManager from the deprecated directory). Thus, when exporting a scene, each entity would be converted into a FileData that a FileManager can then export.

[nmellado]

I would rather use FileIOInterface instead of FileManagerInterface. Otherwise sounds good to me.

[stale[bot]]

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