Enable Explain Code on JH Side

relate with #3200

We are going to enable Explain Code Feature on JH side. This is experimental feature on JH SaaS, only available on group jihulab.com/gitlab-cn. What's more, this feature is highly couple with ChatGLM right now, we will refactor this feature later compatible with other AI providers.

screenshot

How to set up and validate locally

• checkout current-branch
• pass CHAT_GLM_API_KEY=xxx and GITLAB_SIMULATE_SAAS=true as env variable when start rails, I just add variable into gdk/Procfile, for both rails and sidekiq service
• in gdk/Procfile, remember remove SIDEKIQ_QUEUES variable for sidekiq service, to let sidekiq watch all queues
• start gdk services
• Turn on AI feature flags

1. instance level: openai_experimentation, ai_assist_ui, explain_code_snippet
2. group level: ai_related_settings, ai_assist_flag

• create a group, project and add code files under a Ultimate Group

• turn on group AI settings: turn on group -> settings -> general -> permission -> Expermiment features & Third-party AI services