Implement endpoint DELETE /api/v2/manage/customers/{identifier} to delete a customer.

• return 204 in case of success
• create method in the business layer
• GET /api/v2/manage/customers/{identifier} returns 404, after delete
• return an empty body in case of success
• return 404 when the user is not found
• return 403 when user is denied permission
• deprecate POST /manage/customers/delete/<int:client_id>
• openapi: document endpoint
• openapi: deprecate POST /manage/customers/delete/<int:client_id>