Blueprint: GitLab Duo service

What does this MR do and why?

In GitLab AI architecture, we're currently focusing on the enablement side how to activate the Duo features in all GitLab instances. However, the real challenge is to how to develop production-grade AI features that are good enough for production, not ~30% of the time is good enough for a Twitter demo.

This MR adds a blueprint to outline a framework to develop a production-grade AI features with referencing LangChain abstraction.

You can review the doc in https://gitlab.com/gitlab-org/gitlab/-/blob/blueprint-lang-chain/doc/architecture/blueprints/lang_chain/index.md

Related to:

• Prompt Library
• https://www.youtube.com/watch?v=n-stLxM3RSE
• https://docs.gitlab.com/ee/architecture/blueprints/modular_monolith/ #422903