Add GraphQL query for Virtual Registry cleanup policies

📖 Context

This MR is the part of Add GraphQL query and mutations for Virtual Reg... (#577161) • Sylvia Shen • 18.6.

We separated the issue into 2 MRs:

• Add GraphQL query for Virtual Registry cleanup policies 👈🏻 This MR
• Add GraphQL mutation for Virtual Registry cleanup policies

✅ What does this MR do and why?

This MR adds GraphQL query for Virtual Registry cleanup policies to provide a GraphQL equivalent for the REST API GET endpoint.

• Add virtualRegistriesCleanupPolicy query type

  virtualRegistriesCleanupPolicy {
        groupId
        cadence
        createdAt
        updatedAt
        enabled
        keepNDaysAfterDownload
        lastRunAt
        lastRunDeletedSize
        lastRunDetailedMetrics {
          maven {
            deletedSize
            deletedEntriesCount
          }
        }
        lastRunDeletedEntriesCount
        nextRunAt
        status
      }

• Add Resolvers::VirtualRegistries::Cleanup::PolicyResolver
• Add VirtualRegistries::Cleanup::PolicyPolicy

References

N/A

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Switch to this branch 577161-add-graphql-for-vg-cleanup-policies-query
2. Enable feature flag and setup Virtual registry settings

   # Enable maven virtual registry
   ::Feature.enabled(:maven_virtual_registry)
   
   # Create virtual registry setting
   group = Group.find_by(path: 'gitlab-org')
   user = User.find(1)
   VirtualRegistries::Settings::CreateOrUpdateService.new(
     group: group,
     current_user: user,
     params: { enabled: true }
   ).execute
   

3. Create cleanup policy

   VirtualRegistries::Cleanup::Policy.create!(
     group: group,
     keep_n_days_after_download: 30,
     cadence: 7,
     enabled: true,
     notify_on_success: true,
     notify_on_failure: true,
     status: :scheduled,
     last_run_deleted_entries_count: 250,
     last_run_deleted_size: 5120,
     last_run_detailed_metrics: {
       'maven' => {
         'deleted_entries_count' => 150,
         'deleted_size' => 3072
       },
       'container' => {
         'deleted_entries_count' => 100,
         'deleted_size' => 2048
       }
     }
   )

4. Go to Graphql explorer (http://gdk.test:3000/-/graphql-explorer) and test the new query type

    query {
      group(fullPath: "gitlab-org") {
        virtualRegistriesCleanupPolicy {
          groupId
          lastRunAt
          lastRunDeletedSize
          lastRunDetailedMetrics {
            maven {
              deletedSize
              deletedEntriesCount
            }
          }
          lastRunDeletedEntriesCount
        }
      }
    }

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Related to #577161