Add load mode submenu option to OpenRV loaders

[kyadarimahesh]

Changelog Description

• Replace hardcoded load_mode class attribute with EnumDef options
• Add dropdown menu with 'switch' and 'append' modes in loader UI
• Users can now choose load behavior dynamically without code changes
• Applied to both FramesLoader and MovLoader plugins

Additional Review Information

What Changed:

• Replaced hardcoded load_mode = "switch" class attribute with EnumDef options list
• Added import: from ayon_core.lib import EnumDef
• Load logic now reads from options: load_mode = options.get("load_mode", "switch") if options else "switch"

Files Modified:

• client/ayon_openrv/plugins/load/openrv/load_frames.py
• client/ayon_openrv/plugins/load/openrv/load_mov.py

Behavior:

• "switch" (default): Switches to newly loaded source as active view
• "append": Appends to timeline without switching view

Follows the same pattern used in Blender loader plugins

Testing notes:

1. Open AYON Loader in OpenRV
2. Right-click on image sequence or video → Load Frames/Load MOV
3. Verify "Load Mode" dropdown appears with "switch" (default) and "append" options
4. Test "switch" mode: loads and switches to new source as active view
5. Test "append" mode: loads without switching view

Known Issues

⚠️ Icon not showing: The loader icon may not display correctly in some UI contexts (cosmetic issue only, does not affect functionality)