Pioneer: Stop shipping the encryption keys in the library

[motin]

Migrated here from the deprecated pioneer-utils repo: mozilla/pioneer-utils#22

Currently we ship the JOSE public keys (prod/stage) as part of the library. We should come up with a plan to remove these from the library and to instead pulling and caching them from some online source.

[motin]

Pioneer keys can be cycled through remote settings (https://github.com/mozilla/remote-settings)

Basic TODO outline:

• Set up a remote settings collection with the keys
• Make sure that the keys are named / given a uuid in remote settings
• Await keys from remote settings before the start of a Pioneer study
• Annotate pings with metadata about what keys (name/uuid) are used

@rehandalal Comments on this plan?