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/test_validator.rs

@@ -12,6 +12,7 @@ use {
         bank_forks::{ArchiveFormat, SnapshotConfig, SnapshotVersion},
         genesis_utils::create_genesis_config_with_leader_ex,
         hardened_unpack::MAX_GENESIS_ARCHIVE_UNPACKED_SIZE,
+        snapshot_utils::DEFAULT_MAX_SNAPSHOTS_TO_RETAIN,
     },
     solana_sdk::{
         account::{Account, AccountSharedData},
@@ -492,6 +493,7 @@ impl TestValidator {
                 snapshot_package_output_path: ledger_path.to_path_buf(),
                 archive_format: ArchiveFormat::Tar,
                 snapshot_version: SnapshotVersion::default(),
+                maximum_snapshots_to_retain: DEFAULT_MAX_SNAPSHOTS_TO_RETAIN,
             }),
             enforce_ulimit_nofile: false,
             warp_slot: config.warp_slot,