feat(nodes.py): Add support for alpha channel output in LoadImage node

[dzqdzq]

#feature-request

feat(nodes.py): Add support for alpha channel output in LoadImage node

This commit enhances the LoadImage node by adding support for outputting the alpha channel of images. The change ensures that images with transparency can be correctly handled and used in subsequent nodes.

Changes made:

• Modified the load_image method to include an out_alpha parameter.
• Updated the method to check for the alpha channel in the loaded image and output it if requested.

This enhancement allows for more flexible image processing workflows, particularly in scenarios where transparency is crucial.

[Amorano]

#2845

i prefer my explicit version that does not require a switch. you get all three things if they exist as explicit outputs instead of making them pick to output alpha 😁