Add API for uploading and releasing built snaps to the store

[Łukasz Zemczak]

Currently, using Launchpad API, one can manually trigger snap builds out of branches. But once the builds succeed there is no *manual* way to get the generated snap uploaded to the store. There is a way to set up a snap auto-publish rule for the given branch, but sometimes fine-grained control is needed.

Such manual upload and publish methods would be very useful for Bileto snap support, where auto-uploads are not really feasible due to multiple same-snap landings ongoing at once.

[Colin Watson (cjwatson)]

I seem to remember that this may actually have been a misunderstanding of some kind; but regardless, https://launchpad.net/+apidoc/devel.html#snap_build-scheduleStoreUpload is available (and I believe has been available for as long as the rest of snap_build has been exported). Let us know if that's not sufficient.