Enhancement: Include snapshot size in `snapshots` output

The `snapshots` command now prints the size for snapshots created using this
or a future restic version. To achieve this, the `backup` command now stores
the backup summary statistics in the snapshot.

The text output of the `snapshots` command only shows the snapshot size. The
other statistics are only included in the JSON output. To inspect these
statistics use `restic snapshots --json` or `restic cat snapshot <snapshotID>`.

https://github.com/restic/restic/issues/693
https://github.com/restic/restic/pull/4705
https://github.com/restic/restic/pull/4913