Duo Workflow: UI for MCP support settings

What does this MR do and why?

UI for MCP support settings. It's related to MCP support for agentic chat: Model Context Protocol Support for Agentic Chat (#546188 - closed) and https://gitlab.com/gitlab-org/gitlab/-/issues/519938+.

We put the functionality behind a feature flag: [FF] `duo_workflow_mcp_support` -- Control MCP ... (#545956 - closed), but also want to have a setting to control it.

Backend MRs

• Introduce namespace_ai_settings table (!192794 - merged)
• Duo Workflow: Backend for MCP support settings (!193041 - merged)
• Duo Workflow: Define group policy for access check (!193040 - merged)