scylla_node: add ScyllaNode.dump_sstable_stats()

[tchaikov]

ScyllaNode.dump_sstables() is a wrapper around ScyllaNode.run_scylla_sstable(). it provides a more user friendly interface than the latter. it is introduced so that tests can use it with less pain when migrating from node.run_sstablemetadata() to node.run_scylla_sstable().