Publish a public "source of truth" apk push config

[mitchhentges]

This is based off the work laid out in #217, but improves it by removing the need to keep downloading the new "apk push configuration" from taskcluster each time you use the mozapkpublisher CLI. Instead, when build-puppet synchronizes a new APK config, it should publish it to a public location. Then, when mozapkpublisher runs, it should download the config from this public location and use it. This could make mozapkpublisher commands as easy as mozapkpublisher push target.apk nightly --account boxwood-axon@google.com --credentials credentials.p12

Notes:

• Where should this source of truth be? S3 bucket? An endpoint of shipit? 🤔
• When doing a pushapkscript run, do we want to version the "source of truth" it uses? Should re-running the task grab the newest config (like the behaviour now, where you can update pushapkscript and run your task again?) or should it consistently use the same config?