prevent path traversal in s3 to sftp/ftp transfer destinations

[Samin061]

S3ToSFTPOperator and S3ToFTPOperator build the remote destination by concatenating S3 key suffixes returned by list_keys onto the configured sftp_path/ftp_path, so an object whose name contains ../ segments resolves outside that directory on the SFTP/FTP server and the upload lands wherever the server canonicalises it. Object names are arbitrary strings controlled by anyone who can write to the source bucket, so the resolved destination is validated to stay within the configured path before uploading, mirroring the existing gcs_to_sftp and gcs_to_samba guards.

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.