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.

.github/workflows/checks.yml

@@ -29,7 +29,7 @@ jobs:
 
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: "1.65" # MSRV
+          toolchain: "1.70.0" # MSRV
           override: true
 
       - uses: actions-rs/[email protected]
@@ -98,7 +98,7 @@ jobs:
 
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: "1.65" # MSRV
+          toolchain: "1.70.0" # MSRV
           override: true
 
       - uses: actions-rs/[email protected]

.github/workflows/rust-tests.yml

@@ -49,7 +49,7 @@ jobs:
     strategy:
       matrix:
         rust:
-          - "1.65" # MSRV
+          - "1.70.0" # MSRV
           - "stable"
         os:
           - ubuntu-latest