Add aleph alpha api key attribute (#4489)

@tugot17 applied your change to master

langchain/embeddings/aleph_alpha.py

@@ -46,6 +46,8 @@ class AlephAlphaAsymmetricSemanticEmbedding(BaseModel, Embeddings):
     control_log_additive: Optional[bool] = True
     """Apply controls on prompt items by adding the log(control_factor) 
     to attention scores."""
+    aleph_alpha_api_key: Optional[str] = None
+    """API key for Aleph Alpha API."""
 
     @root_validator()
     def validate_environment(cls, values: Dict) -> Dict: