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/utils/cli.py

@@ -57,7 +57,8 @@ default_config = {
         'image':
         {
             'embeddings-file': '.image_embeddings.pt',
-            'batch-size': 50
+            'batch-size': 50,
+            'use-xmp-metadata': 'no'
         },
         'music':
         {