Add node allowing wan 21 generation with start and end frame

### Feature Idea

Actually i saw implementation of this idea with custom nodes, but they relay on kijai wan video wrapper nodes, which i cannot use due to absence of support of scaled checkpoint (while loading normal ones i got OOMs). Thus i want the support of this feature on in native comfyui environment.

### Existing Solutions

Heres the repo of existing solution
https://github.com/raindrop313/ComfyUI-WanVideoStartEndFrames

### Other

_No response_