LangchainProcessor can not use the OpenAILLMContext

[markbackman]

Description

The LangchainProcessor is out of date with the rest of Pipecat in terms of working with the context. There are two issues:

• The LangchainProcessor relies on LLMUserResponseAggregator and LLMAssistantResponseAggregator, which are deprecated.
• The LangchainProcessor is not a subclass of an AIService (like other LLM services), so it cannot use the OpenAILLMContext, which makes it an outlier from other services.

Work to be completed

This issue is tracking the work to align the LangchainProcessor with how the other services use the OpenAILLMContext to track the context and use tools.