api: filter snapshot counts

unprivileged users should only see the counts related to their part of
the datastore.

while we're at it, switch to a list groups, filter groups, count
snapshots approach (like list_snapshots) to speedup calls to this
endpoint when many unprivileged users share a datastore.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

src/api2/types/mod.rs

@@ -692,7 +692,7 @@ pub struct TypeCounts {
         },
     },
 )]
-#[derive(Serialize, Deserialize)]
+#[derive(Serialize, Deserialize, Default)]
 /// Counts of groups/snapshots per BackupType.
 pub struct Counts {
     /// The counts for CT backups