Add clusterAgents field to Project/Group/Security Dashboard GraphQL API

What does this MR do and why?

We're adding ability to filter vulnerabilities based on Kubernetes Agent, so you will be able to see vulnerabilities created for selected cluster (using cluster-image-scanning capabilities).

Queries

Group query

explain SELECT "cluster_agents".* FROM "cluster_agents" WHERE "cluster_agents"."project_id" IN (SELECT "projects"."id" FROM "projects" WHERE "projects"."namespace_id" IN (WITH RECURSIVE "base_and_descendants" AS ((SELECT "namespaces"."id" FROM "namespaces" WHERE "namespaces"."type" = 'Group' AND "namespaces"."id" = 9970)
UNION
(SELECT "namespaces"."id" FROM "namespaces", "base_and_descendants" WHERE "namespaces"."type" = 'Group' AND "namespaces"."parent_id" = "base_and_descendants"."id")) SELECT "namespaces"."id" FROM "base_and_descendants" AS "namespaces"))

Summary:
Time: 531.206 ms
  - planning: 9.113 ms
  - execution: 522.093 ms
    - I/O read: 491.574 ms
    - I/O write: 0.000 ms

Shared buffers:
  - hits: 10216 (~79.80 MiB) from the buffer pool
  - reads: 1356 (~10.60 MiB) from the OS file cache, including disk I/O
  - dirtied: 186 (~1.50 MiB)
  - writes: 0

Details and visualization: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/10542/commands/37862.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to GraphQL filter for vulnerabilities by container... (#358763 - closed)

ee/spec/models/instance_security_dashboard_spec.rb

@@ -221,7 +221,17 @@
     let_it_be(:cluster_agent_for_project_1) { create(:cluster_agent, project: project1) }
     let_it_be(:cluster_agent_for_project_3) { create(:cluster_agent, project: project3) }
 
-    it { expect(instance_dashboard.cluster_agents).to contain_exactly(cluster_agent_for_project_1) }
+    context 'when instance security dashboard has projects added' do
+      it { expect(instance_dashboard.cluster_agents).to contain_exactly(cluster_agent_for_project_1) }
+    end
+
+    context 'when instance security dashboard does not have any projects added' do
+      let_it_be(:other_user) { create(:user) }
+
+      subject(:instance_dashboard) { described_class.new(other_user, project_ids: []) }
+
+      it { expect(instance_dashboard.cluster_agents).to be_empty }
+    end
   end
 
   describe '#full_path' do