Create config_sentence_transformers.json

Adding a config for sentence-transformers to automatically encode with "query: " and "passage: " prompt prefixes.

config_sentence_transformers.json

@@ -0,0 +1,5 @@
+{
+  "prompts": {"query": "query: ", "passage": "passage: "},
+  "default_prompt_name": null,
+  "similarity_fn_name": "cosine"
+}