Introduce namespace_ai_settings table

What does this MR do and why?

Introduce namespace_ai_settings table. It'll be used to store AI-related settings per namespace

• Currently, we store the AI-related settings in the general namespace_settings table: duo_features_enabled, lock_duo_features_enabled, duo_nano_features_enabled, enable_auto_assign_gitlab_duo_pro_seats
• We're considering adding a setting to control MCP support per namespace: duo_workflow_mcp_support
• There'll be definitely more namespace (and project) settings related to AI

The proposal is to create a separate table for AI-related settings instead of extending the general one (which is large already). It seems that we introduced namespace_ci_cd_settings and project_ci_cd_settings for the same purposes.