bin/show: Add --print-sizes option to show sizes metadata

Use the new `ostree_commit_get_object_sizes()` API to read the
`ostree.sizes` commit metadata and print a summary.

bash/ostree

@@ -1445,6 +1445,7 @@ _ostree_show() {
     local boolean_options="
         $main_boolean_options
         --print-related
+        --print-sizes
         --raw
     "