refactor: simplify langchain/llm configs

[navalnica]

StatGPT Backend version

latest

What is the problem this feature will solve?

there is a separation of concerns issue between langchain settings and llm/embedding models settings:

• langchain settings allow to set default llm/embedding params via envvars (e.g. temperature)
• this does not introduce much value (we likely don't have a need to control such params via envvars, since we've moved to comprehensive llm models configs)
• however, it makes settings complex and unclear. it's much clearer to control these params using llm/embedding models configs

What is the proposed feature or solution?

• remove following LangChainSettings fields:
  • default_model (? could be a large refactoring and might be not worth it. but I think it's a good idea to explicitly specify the LLM model to use)
  • embedding_default_model (??? even larger refactoring needed - probably keep it as is)
  • default_temperature
  • default_seed
  • remove corresponding envvars, upd envvars markdown listing
• set api_version in langchain config, remove it from llm/embeddings model config

after removing envvars we'll need to update

• envvars readme files
• helms
• .env.template

What alternatives have you considered?

keep as is

[Fedir-Yatsenko]

Good suggestion. Just discovered that we are NOT setting default_seed environment variable for the LangChainSettings in the admin_backend. But we set this up for chat:
https://github.com/epam/statgpt-helm/blob/main/charts/statgpt/values.yaml#L96

This is another argument in favor of solving this problem.