Add Graphql endpoints for Duo Settings

What does this MR do and why?

This MR adds Graphql endpoints for interfacing with the ai_settings table. This will support the work here for adding UI functionality to update the AI Gateway URL. Currently it can only be set by using the ENV variable AI_GATEWAY_URL or by updating ::Ai::Setting in the rails console, but we need a way for the frontend to be able to interact with this value.

This MR adds:

• A query endpoint for Duo settings
• An update mutation + service for updating Duo settings

References

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Before	After

How to set up and validate locally

Prequisites

• You will need an active GitLab Ultimate license.
• Make sure you are set up with a Duo Enterprise subscription by running the following command in the terminal:

GITLAB_SIMULATE_SAAS=0 bundle exec 'rake gitlab:duo:setup_instance[duo_enterprise]'

Test it!

1. Start GDK (gdk start) and visit http://gdk.test:3000/-/graphql-explorer

2. Test the Duo settings query by running the following query in the graphql explorer

query getDuoSettings {
  duoSettings {
    aiGatewayUrl
  }
}

You should expect a response that looks something like the below:

{
  "data": {
    "aiGatewayUrl": "<YOUR_AI_GATEWAY_URL>"
  },
  "correlationId": "01JHSNX0Y2VZJ6E0R9NPBYNRVX"
}

3. Test the Duo settings update mutation by running the following in the graphql explorer

mutation updateDuoSettings($input: DuoSettingsUpdateInput!) {
  duoSettingsUpdate(input:$input) {
    aiGatewayUrl 
    errors
  }
}

In the variables tab add the following:

{
  "input": {
    "aiGatewayUrl": "http://0.0.0.0:8080"
  }
}

You should expect a response that looks something like the below:

{
  "data": {
    "duoSettingsUpdate": {
      "duoSettings": {
        "aiGatewayUrl": "http://0.0.0.0:8080"
      },
      "errors": []
    }
  },
  "correlationId": "01JHSP4QKATXBGEEF3PWFD0T3F"
}

Related to #473143 (closed)