* Add support for cloud sync, specifically GCP
This adds generic support for sync to cloud services, with specific
spuport for GCP. Adding others -- so long as they support a
compare-and-set operation -- should be comparatively straightforward.
The cloud support includes cleanup of unnecessary data, and should keep
total space usage roughly proportional to the number of tasks.
Co-authored-by: ryneeverett <ryneeverett@gmail.com>
In #3118 @ryneeverett mentioned that "key" suggests that this is a
secret, when in truth it's just a user identifier. So "ID" is a better
word for it than "key".
