[AUR-389] Add base interface and embedding model (#17)

This change provides the base interface of an embedding, and wrap the Langchain's OpenAI embedding. Usage as follow:

```python
from kotaemon.embeddings import AzureOpenAIEmbeddings

model = AzureOpenAIEmbeddings(
    model="text-embedding-ada-002",
    deployment="embedding-deployment",
    openai_api_base="https://test.openai.azure.com/",
    openai_api_key="some-key",
)
output = model("Hello world")
```

knowledgehub/embeddings/openai.py

@@ -0,0 +1,15 @@
+from langchain.embeddings import OpenAIEmbeddings as LCOpenAIEmbeddings
+
+from .base import LangchainEmbeddings
+
+
+class OpenAIEmbeddings(LangchainEmbeddings):
+    _lc_class = LCOpenAIEmbeddings
+
+
+class AzureOpenAIEmbeddings(LangchainEmbeddings):
+    _lc_class = LCOpenAIEmbeddings
+
+    def __init__(self, **params):
+        params["openai_api_type"] = "azure"
+        super().__init__(**params)