Get the list of all client applications attached to the user's account.

This method returns a list of `AttachedClient` structs representing all the applications connected to the user's account. This includes applications that are registered as a device as well as server-side services that the user has connected.

This information is really only useful for targeted messaging or marketing purposes, e.g. if the application wants to advertize a related product, but first wants to check whether the user is already using that product.

Notes

- Attached client metadata is only visible to applications that have been
  granted the `https:///identity.mozilla.com/apps/oldsync` scope.