datastore: implement Iterator for backup group listing

While currently it's still only used in a collected() way, most call
sites can be switched over to use the iterator directly, as often
they already convert the not-so-cheap, in-memory vector back in
.into_iter() anyway.

somewhat also preparatory (yak shaving) work for namespaces

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

pbs-datastore/src/lib.rs

@@ -204,7 +204,7 @@ pub use manifest::BackupManifest;
 pub use store_progress::StoreProgress;
 
 mod datastore;
-pub use datastore::{check_backup_owner, DataStore};
+pub use datastore::{check_backup_owner, DataStore, ListGroups};
 
 mod snapshot_reader;
 pub use snapshot_reader::SnapshotReader;