Create apis for instance audit events

What does this MR do and why?

This MR adds a graphql api for creating instance level audit event streaming destinations as per the part of consolidation of apis efforts in &12339. This API is similar to the one created for group level destinations in !147888 (merged).

The code diff adds a new feature that allows administrators to create external destinations to stream instance-level audit events. This feature is disabled by default and requires a license to be enabled. Only administrators can create these destinations, and the action is audited. The code also includes tests to ensure the feature works as expected.

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Before	After

How to set up and validate locally

1. For testing this, you need to have a GitLab instance with Ultimate license.
2. Visit graphql explorer, for example http://gitlab.localdev:3000/-/graphql-explorer on your local setup.
3. Add following mutation for creating a new instance level external audit event destination.

mutation instanceAuditEventStreamingDestinationsCreate {
  instanceAuditEventStreamingDestinationsCreate(input: {
    name: "testdest5",
    category: "http",
    secretToken: "random_secret_token",
    config: {
      url: "https://www.example.com"
    }
  }) {
    errors
    externalAuditEventDestination {
      id
      name
      config
      category
    }
  }
}

4. You should receive a destination object in response, something like, which means that the destination has been created.

{
  "data": {
    "instanceAuditEventStreamingDestinationsCreate": {
      "errors": [],
      "externalAuditEventDestination": {
        "id": "gid://gitlab/AuditEvents::Instance::ExternalStreamingDestination/1",
        "name": "testdest5",
        "config": {
          "url": "https://www.example.com"
        },
        "category": "http"
      }
    }
  }
}

5. You can also try by giving config as a string, or not passing required parameters, all of these should return errors.
6. For verifying creation of the destination, you can also cross check in rails console by running AuditEvents::Instance::ExternalStreamingDestination.all

Related to #436615 (closed)