add a voucher_subscriber option

[Nilay-Shah]

introduce a third option to run Voucher: voucher_subscriber. this should implement the existing Server interface to allow us to hook into a GCP pub/sub topic to vouch images automatically. since there is already a topic to subscribe to for GCR images, this allows us to run Voucher and automatically vouch all images pushed to GCR.
the project and subscription will be passed in as command line arguments (ie. voucher_subscriber --project=my-cool-project --subscription=all-gcr-images

cc: @ooq @cjones-shopify

[ook]

Interesting typo ping. And interesting project. Have fun 👍 ;)