feat: Option to use last existing snapshot

[kichik]

Instead of creating a new snapshot, set useExistingSnapshot to true and use the latest available snapshot. This can speed up the process significantly, but may end up using older than desired snapshots. If you already have an automated snapshot, it might be good enough.

new RdsSanitizedSnapshotter(sfnStack, 'MySQL Cluster Snapshotter', {
  vpc,
  databaseCluster: mysqlDatabaseCluster,
  script: 'SELECT 1',
  snapshotPrefix: 'mysql-cluster-snapshot',
  useExistingSnapshot: true, // <====
});

Resolves #1