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)

app/finders/clusters/agents_finder.rb

@@ -4,8 +4,8 @@ module Clusters
   class AgentsFinder
     include FinderMethods
 
-    def initialize(project, current_user, params: {})
-      @project = project
+    def initialize(object, current_user, params: {})
+      @object = object
       @current_user = current_user
       @params = params
     end
@@ -13,7 +13,7 @@ def initialize(project, current_user, params: {})
     def execute
       return ::Clusters::Agent.none unless can_read_cluster_agents?
 
-      agents = project.cluster_agents
+      agents = object.cluster_agents
       agents = agents.with_name(params[:name]) if params[:name].present?
 
       agents.ordered_by_name
@@ -21,10 +21,10 @@ def execute
 
     private
 
-    attr_reader :project, :current_user, :params
+    attr_reader :object, :current_user, :params
 
     def can_read_cluster_agents?
-      current_user.can?(:read_cluster, project)
+      current_user&.can?(:read_cluster, object)
     end
   end
 end