wip: First stab at langchain support

Is this a service or processor?
How to deal with conversation history? LC has sophisticated means of this, but might get in the way of `LLMResponseAggregator`

pyproject.toml

@@ -40,6 +40,7 @@ examples = [ "python-dotenv~=1.0.0", "flask~=3.0.3", "flask_cors~=4.0.1" ]
 fal = [ "fal-client~=0.4.0" ]
 google = [ "google-generativeai~=0.5.3" ]
 fireworks = [ "openai~=1.26.0" ]
+langchain = [ "langchain~=0.2.1" ]
 local = [ "pyaudio~=0.2.0" ]
 moondream = [ "einops~=0.8.0", "timm~=0.9.16", "transformers~=4.40.2" ]
 openai = [ "openai~=1.26.0" ]