Add remote api call for quit.

[mikewolfxyou]

This feature could be used when we need to send a quit signal to shut down the service from outside of the push gateway container.

e.g., a Kubernetes cron job uses the push gateway to push the metric; when the corn job is finished, we need to send a shutdown signal to the push gateway container otherwise, the push gateway container will keep running forever, and the job will be marked as failed.