Add REST API endpoint for designating centralised security policy management group (CSP)

What does this MR do and why?

We are working on centralised security policy management (CSP, &17392 (closed)). We need a mechanism to designate a top-level group as the instance's centralised security policy group, for which this MR implements a new REST API endpoint PUT admin/security/policy_settings.

References

#541511 (closed)

How to set up and validate locally

Use an administrator's $GITLAB_TOKEN and verify that ::Security::PolicySetting.instance.csp_namespace_id reflects the updates.

Setting the CSP group

curl -X PUT "http://gdk.test:3000/api/v4/admin/security/policy_settings" \
  -H "Authorization: Bearer $GITLAB_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"csp_namespace_id": 123456}'

Unsetting the CSP group

curl -X PUT "http://gdk.test:3000/api/v4/admin/security/policy_settings" \
  -H "Authorization: Bearer $GITLAB_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"csp_namespace_id": null}'

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Related to #541511 (closed)