Changes to Lists the customers holding relations on one app, one cursor page at a time.

added the new Headscale enum value to the items/items/app response property for the response status 200

added the new User enum value to the items/items/relation response property for the response status 200

added the new enum value Headscale to the query request parameter app

added the new enum value User to the query request parameter relation

the security scope api was added to the endpoint's security scheme OpenIdConnect

endpoint added