Make Using XMP Metadata to Enhance Image Search Optional, Configurable

- Break the compute embeddings method into separate methods:
  compute_image_embeddings and compute_metadata_embeddings

- If image_metadata_embeddings isn't defined, do not use it to enhance
  search results. Given image_metadata_embeddings wouldn't be defined
  if use_xmp_metadata is False, we can avoid unnecessary addition of
  args to query method

src/main.py

@@ -189,6 +189,7 @@ if __name__ == '__main__':
             pathlib.Path(image_config['embeddings-file']),
             batch_size=image_config['batch-size'],
             regenerate=args.regenerate,
+            use_xmp_metadata={'yes': True, 'no': False}[image_config['use-xmp-metadata']],
             verbose=args.verbose)
 
     # Start Application Server