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

sample_config.yml

@@ -12,6 +12,7 @@ content-type:
   image:
     embeddings-file: '.image_embeddings.pt'
     batch-size: 50
+    use-xmp-metadata: 'no'
 
 search-type:
   asymmetric: