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

multinode-demo/bootstrap-validator.sh

@@ -72,6 +72,9 @@ while [[ -n $1 ]]; do
     elif [[ $1 == --accounts ]]; then
       args+=("$1" "$2")
       shift 2
+    elif [[ $1 == --maximum-snapshots-to-retain ]]; then
+      args+=("$1" "$2")
+      shift 2
     else
       echo "Unknown argument: $1"
       $program --help

multinode-demo/validator.sh

@@ -119,6 +119,9 @@ while [[ -n $1 ]]; do
     elif [[ $1 = --snapshot-interval-slots ]]; then
       args+=("$1" "$2")
       shift 2
+    elif [[ $1 = --maximum-snapshots-to-retain ]]; then
+      args+=("$1" "$2")
+      shift 2
     elif [[ $1 = --limit-ledger-size ]]; then
       args+=("$1" "$2")
       shift 2