Br restore should prevent users from restoring to a database or tables that already exists

## Enhancement
Try backup and restore as follows, BR restore failed at the final checksum stage, but all SST files have been injected, which may triage compaction in TiKV, as described in issue https://github.com/tikv/tikv/issues/14540.

tiup br:v7.0.0 backup full --pd 172.16.4.36:2379 --storage "s3://prod-cluster-do-not-remove/backups/20230407-full" --log-file /tmp/backup-full-0407-local-minio.log

/br restore full --pd 172.16.4.36:2379 --storage "s3://prod-cluster-do-not-remove/backups/20230407-full" --ratelimit 128

BR should support checking that before starting the import, and it would be even better if it also supports an option to ignore this check.