(#10289) Add a safe alternative to REST for inventory service

With the default implementation of the inventory service, with a
terminus REST and cache YAML, a failed upload to the inventory service
would cause compilation to fail. This means the inventory service was a
single point of failure for the entire Puppet infrastructure. Now, we
introduce an inventory_service terminus which can be used in place of
the REST terminus, and will absorb failures, allowing compilation to
continue.