Add context windows callback for custom cond handling

[drozbay]

Add a callback handler for the resize_cond_item function that can be used to perform custom slicing of any conditioning item for proper use in context windows.

[drozbay]

An example implementation of the use of these hooks can be found in my nodes ComfyUI-WanVaceAdvanced.

Find the workflow itself here

https://github.com/user-attachments/assets/077fb98f-5926-4867-8c21-6393c5d78e75