datastore: move list_backup_groups into Datastore impl

Having that as static method in BackupInfo makes zero sense and just
complicates call sites, which need to extract the base_path from the
store manually upfront.

Mark old fn as deprecated so that we can do the move in a separate
step.

It's also planned to add an Iterator impl for this to allow more
efficient usage in the future.

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

pbs-datastore/src/backup_info.rs

@@ -369,6 +369,7 @@ impl BackupInfo {
         Ok(files)
     }
 
+    #[deprecated = "move to datastore"]
     pub fn list_backup_groups(base_path: &Path) -> Result<Vec<BackupGroup>, Error> {
         let mut list = Vec::new();