Add filtering for confidential issues for GraphQL API

What does this MR do and why?

Resolves #327344 (closed).

• Adds confidential boolean argument to the issues resolver to allow filtering by confidentiality state.

Note that the REST API already exists and this MR mimics the feature for GraphQL.

Sample query 1:

{
  group(fullPath: "gitlab-org") {
    issues(confidential: true) {
      nodes {
        id
        confidential
      }
    }
  }
}

Sample query 2:

{
  project(fullPath: "gitlab-org/gitlab-shell") {
    issues(confidential: true) {
      nodes {
        id
        confidential
      }
    }
  }
}

MR acceptance checklist

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