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

core/src/validator.rs

@@ -626,6 +626,7 @@ impl Validator {
                     snapshot_hash,
                     &exit,
                     &cluster_info,
+                    snapshot_config.maximum_snapshots_to_retain,
                 );
                 (
                     Some(snapshot_packager_service),
@@ -1166,6 +1167,7 @@ fn new_banks_from_ledger(
             &snapshot_config.snapshot_package_output_path,
             snapshot_config.archive_format,
             Some(&bank_forks.root_bank().get_thread_pool()),
+            snapshot_config.maximum_snapshots_to_retain,
         )
         .unwrap_or_else(|err| {
             error!("Unable to create snapshot: {}", err);