Added create API for group level external audit event destination

What does this MR do and why?

This MR does following:

1. Adds create API for newly added consolidated external audit event streaming destinations for top-level groups, as part of &12339.
2. Lists the new consolidated audit event destinations associated with a group.

Please note that the config param is just expected to be a json for now, but there are separate issues #436607, #436608 and #436609 for adding handling of validations for various different types of destinations.

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Before	After

How to set up and validate locally

1. For testing this, you need to have a group 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 external destination, I am creating one for a group named twitter but you can pass any path for a top-level group.

mutation groupAuditEventStreamingDestinationsCreate {
  groupAuditEventStreamingDestinationsCreate(input: {
    name: "testdest4",
    category: "http",
    groupPath: "twitter",
    secretToken: "asfergrbtr",
    config: {
      url: "https://www.example.com"
    }
  }) {
    errors
    externalAuditEventDestination {
      id
      name
      config
      category
      group {
        id
        name
      }
    }
  }
}

4. You should receive a destination object in response, something like.

{
  "data": {
    "groupAuditEventStreamingDestinationsCreate": {
      "errors": [],
      "externalAuditEventDestination": {
        "id": "gid://gitlab/AuditEvents::Group::ExternalStreamingDestination/7",
        "name": "testdest4",
        "config": {
          "url": "https://www.example.com"
        },
        "category": "http",
        "group": {
          "id": "gid://gitlab/Group/35",
          "name": "Twitter"
        }
      }
    }
  }
}

5. You can also try by giving config as a string, or not passing required parameters, or by providing path to a subgroup, all of these should return errors.
6. For fetching list of all external destinations belonging to a group, run the following query:

query groupDestinations {
  group(fullPath: "twitter") {
    id
    name
    externalAuditEventStreamingDestinations {
      nodes {
        id
        name
        category
      }
    }
  }
}

Related to #436610 (closed)