tools/scylla-sstable: add alternative schema load method for system tables
tools/scylla-sstable: add alternative schema load method for system tables

By providing its name via the new `--system-schema` option. The schema
will be loaded from the internal hardcoded definition.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla