Expose securityPolicyProject for project or group in GraphQL API

What does this MR do and why?

This MR exposes securityPolicyProject for project or group in GraphQL API.

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Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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Project	Group

How to set up and validate locally

1. Create a new group
2. Visit Secure -> Policies and create a group policy. The security policy project will be created automatically with it. Sample policy YAML:

   type: scan_result_policy
   name: Test policy
   description: ''
   enabled: true
   rules:
     - type: any_merge_request
       branch_type: protected
       commits: any
   actions:
     - type: require_approval
       approvals_required: 1
       role_approvers:
         - developer

3. Query GraphQL (/-/graphql-explorer):

   query {
     namespace: group(fullPath: "<group-path>") {
       id
       securityPolicyProject {
         name
         fullPath
         openMergeRequestsCount
       }
     }
   }

4. Create a new project
5. Visit Secure -> Policies and create a project policy. The security policy project will be created automatically with it. Sample policy YAML:

   type: scan_result_policy
   name: Test policy
   description: ''
   enabled: true
   rules:
     - type: any_merge_request
       branch_type: protected
       commits: any
   actions:
     - type: require_approval
       approvals_required: 1
       role_approvers:
         - developer

6. Query GraphQL (/-/graphql-explorer):

   query {
     namespace: project(fullPath: "<project-path>") {
       id
       securityPolicyProject {
         name
         fullPath
         openMergeRequestsCount
       }
     }
   }

Related to #414988 (closed)