Expose runbook in alert GraphQL API

What does this MR do?

Exposes a runbook attribute on AlertManagement::Alert in the graphql API. This field will often be a URL, but it could be possible for a user to put anything in this field, as alert payloads are externally defined.

This change will make it easier for the frontend to display the runbook content from the alert detail overview.

Related issue: #220400 (closed)

Testing

1. Generate alerts with a runbook from the rails console or through the alert-integration test UI (Operations > Alerts)

AlertManagement::Alert.create!(
  project_id: <REPLACE ME WITH A PROJECT ID (ex 21)>,
  payload: {"startsAt"=>"#{Time.now.rfc3339}Z", "runbook" => { 'step1' => 'fix it', 'step2' => 'update stakeholders'}}},
  title: 'Alert with non-url runbook',
  started_at: Time.now
)
AlertManagement::Alert.create!(
  project_id: <REPLACE ME WITH A PROJECT ID (ex 21)>,
  payload: {"startsAt"=>"#{Time.now.rfc3339}Z", "runbook" => "https://gitlab.com" },
  title: 'Alert with a url runbook',
  started_at: Time.now
)

2. Run a graphql query like this one under /-/graphql-explorer:

{
  project(fullPath: "path/for/your/project") {
    id
    fullPath
    alertManagementAlerts {
      nodes {
        iid
	status
	title
        runbook
      }
    }
  }
}

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team