Add blueprints for Langchain

[anna-geller]

Langchain is now merged with a 2nd PR ready https://github.com/kestra-io/plugin-langchain/pull/2/files

We should:

• Evaluate if the plugin design makes sense - @Ben8t maybe you can have a look, seems like a good opportunity to learn about Langchain 💪 + check if all property naming + tasks overall make sense and are usable
• QA if it works as expected (for those we can test, I don't expect us to run OSS models on our laptop 🙃 )
• Add some blueprints - @aj-emerich