Output parameters generated as `const` references

### Godot version

4.2-stable

### godot-cpp version

4.2-stable

### System information

Godot v4.2.1.stable - macOS 14.4.1 - Vulkan (Forward+) - integrated Apple M1 - Apple M1 (8 Threads)

### Issue description

The generated interface for both `EditorImportPlugin::_import` and `EditorResourcePreviewGenerator::_generate` have const-qualified parameters that are actually intended to be modified.

In the docs for `EditorImportPlugin::_import`, the `platform_variants` and `gen_files` parameters are suppose to be lists that can be used to inform the editor of additional files related to the import. Because they're const-qualified in the godot-cpp interface they can't be updated without a `const_cast`.


Same goes for `EditorResourcePreviewGenerator::_generate`'s `metadata ` parameter which is suppose to updated by the preview generator to provide any applicable metadata to `EditorResourceTooltipPlugin::_make_tooltip_for_path` for the editor tooltip.


### Steps to reproduce

N/A

### Minimal reproduction project

N/A