Add the capability to trigger Duo flow execution on the AI catalog

What does this MR do and why?

This MR adds a mutation to execute Duo flow. Currently, the Duo workflow service does not have the ability to receive flow YAML configuration via API. Therefore, this mutation will return a YAML-based string, and we need to copy and paste it into the Duo workflow service manually in order to execute it.

Input Parameters:

• agentId (required)
• versionId (optional) - If provided, the specified agent version will be used; otherwise, the latest version will be used.

mutation {
  aiCatalogAgentExecute(input: {
    agentId: "gid://gitlab/Ai::Catalog::Item/41",
    agentVersionId: "gid://gitlab/Ai::Catalog::ItemVersion/40"   
  }) {
    errors
    flowConfig
  }
}

Response :

{
  "data": {
    "aiCatalogAgentExecute": {
      "errors": [],
      "flowConfig": "---\nversion: experimental\nenvironment: remote\ncomponents:\n- name: '41'\n  type: AgentComponent\n  prompt_id: workflow_catalog\n  prompt_version: \"^1.0.0\"\n  inputs:\n  - context:goal\n  output: context:agent.answer\n  toolset:\n  - gitlab_blob_search\n  - ci_linter\n  - gitlab_commit_search\n  - edit_file\n  ui_log_events:\n  - on_tool_execution_success\n  - on_agent_final_answer\n  - on_tool_execution_failed\nrouters:\n- from: '41'\n  to: end\nflow:\n  entry_point: '41'\n"
    }
  },
  "correlationId": "01K2CHTYR8J2GKV11J4V7QM7BF"
}

This site can be used to format YAML config - https://dadroit.com/string-to-yaml/

Related to #560129 (closed)