Copy-paste current implementation of core LoadImage, but use a string
input providing an entity reference instead of a file upload, and
retrieve input file path from the OpenAssetIO manager.
Due to the mix-and-match by ComfyUI of class vs. instance methods, plus
no way to inject context, use a singleton class to represent the
OpenAssetIO host application.
Signed-off-by: David Feltell <david.feltell@foundry.com>