datastore: add snapshot iterator and provide example

will be more used in the future, when the upend-datastore master plan
comes in effect.

also a preparatory work for namespaces

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

pbs-datastore/examples/ls-snapshots.rs

@@ -0,0 +1,34 @@
+use std::path::PathBuf;
+
+use anyhow::{bail, Error};
+
+use pbs_datastore::{ListGroups, ListSnapshots};
+
+fn run() -> Result<(), Error> {
+    let base: PathBuf = match std::env::args().skip(1).next() {
+        Some(path) => path.into(),
+        None => bail!("no path passed"),
+    };
+
+    for group in ListGroups::new(base.to_owned())? {
+        let group = group?;
+        println!("found group {}", group);
+
+        let group_path = base.as_path().join(group.to_string());
+        for snapshot in ListSnapshots::new(group, group_path)? {
+            println!("\t{}", snapshot?);
+        }
+    }
+
+    Ok(())
+}
+
+fn main() {
+    std::process::exit(match run() {
+        Ok(_) => 0,
+        Err(err) => {
+            eprintln!("error: {}", err);
+            1
+        }
+    });
+}