Add support for cloud sync, specifically GCP (#3223)

* 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>

src/commands/CmdShow.cpp

@@ -193,6 +193,7 @@ int CmdShow::execute (std::string& output)
     " sugar"
     " summary.all.projects"
     " sync.local.server_dir"
+    " sync.gcp.bucket"
     " sync.server.client_id"
     " sync.server.encryption_secret"
     " sync.server.origin"