Expose an alert's (global) ID in GraphQL queries

What does this MR do and why?

Adds an alert global ID to GraphQL queries.

Related to #356057 (closed)

Why do we need to expose a global alert ID when we already exposing an Internal ID?

It will be possible to link external alerts to an incident. An external alert, in this case, is an alert that does not belong to the same project as an incident. When we want to unlink such an alert from the incident, knowing the alert's internal ID is not enough.

Screenshots or screen recordings

GraphQL query

query Issue {
  issue(id:"gid://gitlab/Issue/441") {
    id
    title
    alertManagementAlerts {
      nodes {
        id
        title
      }
    }
  }
}

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