Add Huggingface embeddings and Cohere embeddings (#63)

* Add huggingface embeddings and cohere embeddings
* Update openai interface and the mock for newer OpenAI SDK

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Co-authored-by: trducng <trungduc1992@gmail.com>

knowledgehub/embeddings/cohere.py

@@ -0,0 +1,12 @@
+from langchain.embeddings import CohereEmbeddings as LCCohereEmbeddings
+
+from kotaemon.embeddings.base import LangchainEmbeddings
+
+
+class CohereEmbdeddings(LangchainEmbeddings):
+    """Cohere embeddings.
+
+    This class wraps around the Langchain CohereEmbeddings class.
+    """
+
+    _lc_class = LCCohereEmbeddings