Issue #17008 -- make snapshot archives to hold on to configurable. (#17158)

* purge_old_snapshot_archives is changed to take an extra argument 'maximum_snapshots_to_retain' to control the max number of latest snapshot archives to retain. Note the oldest snapshot is always retained as before and is not subjected to this new options.
* The validator and ledger-tool executables are modified with a CLI argument --maximum-snapshots-to-retain. And the options are propagated down the call chains. Their corresponding shell scripts were changed accordingly.
* SnapshotConfig is modified to have an extra field for the maximum_snapshots_to_retain
* Unit tests are developed to cover purge_old_snapshot_archives

local-cluster/tests/local_cluster.rs

@@ -1684,6 +1684,7 @@ fn test_snapshot_download() {
         &validator_archive_path,
         archive_snapshot_hash,
         false,
+        snapshot_utils::DEFAULT_MAX_SNAPSHOTS_TO_RETAIN,
     )
     .unwrap();
 
@@ -3134,6 +3135,7 @@ fn setup_snapshot_validator_config(
         snapshot_path: PathBuf::from(snapshot_dir.path()),
         archive_format: ArchiveFormat::TarBzip2,
         snapshot_version: snapshot_utils::SnapshotVersion::default(),
+        maximum_snapshots_to_retain: snapshot_utils::DEFAULT_MAX_SNAPSHOTS_TO_RETAIN,
     };
 
     // Create the account paths