Langsmith prompt templates

[mbertrand]

What are the relevant tickets?

Part of https://github.com/mitodl/hq/issues/7192.
The tutorbot prompts will be handled later.

Description (What does it do?)

• Adds a new optional setting, LANGSMITH_API_KEY
• Modifies the way system prompts are assigned to chatbots:
  a) Get from hardcoded values if LangSmith is not enabled; otherwise:
  b) Get from the cache if present
  c) Get from langsmith if present
  d) Create and return the prompt value in langsmith if not already present, and set the cache too.

How can this be tested?

• Try out the various AI tabs without LANGSMITH_API_KEY set, the chatbots should work same as before.
• Create a free personal langsmith account at https://smith.langchain.com/
• Obtain an API key and add it to your .env file as LANGSMITH_API_KEY
• Restart containers and try out the chatbots again, they should still work.
• In the langsmith UI, choose the "Prompts" tool, You should see prompts for the chatbots you tried.