Enhancement: Add $image_classes parameter to ResponsivePics::get_picture()

[huubl]

Is your feature request related to a problem? Please describe.
The current implementation of the ResponsivePics::get_picture() function within the ResponsivePics WordPress plugin lacks the functionality to apply custom CSS classes to the individual <img> elements that it generates. This limitation makes it challenging to integrate ResponsivePics::get_picture() with Gutenberg blocks such as core/image or core/cover, which rely on specific CSS classes for styling.

Describe the solution you'd like
I propose adding an optional parameter to the ResponsivePics::get_picture() function. This new parameter would allow developers to specify CSS classes that should be applied to the individual <img> elements within the <picture> element.

Describe alternatives you've considered
Non

Additional context
Non