Support "push" mode for voucher subscriber

[ooq]

Voucher subscriber currently supports a pull-based pub-sub subscription, i.e., the voucher subscriber will periodically pull for new messages.

An alternative mode is "push", where a subscription sends new messages to an http endpoint behind which the voucher subscriber runs.

Or, this feature should be added to voucher server instead?

Motivation:
The current pull based subscriber cannot be packaged to run on Google Cloud Run (or other microservice platform), due to its "serverful" nature.

cc @Nilay-Shah @cjones-shopify for comments.

[Nilay-Shah]

this is a good point; I don't think we considered the ability to have this run on serverless.

i think this makes the most sense to be part of the subscriber because this is another method of pub/sub. if i were using this package for the first time i'd expect something like voucher_subscriber push ... to work instead of having this functionality being a part of voucher_server ... even though the actual code for the push pub/sub option is pretty much the same as the actual Voucher server!