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**Description**

I want to specify the cipher to use when creating a SftpHook. Currently this is not possible because there is not a way to propagate the value to CnOpts. 

https://github.com/apache/airflow/blob/master/airflow/providers/sftp/hooks/sftp.py
https://pysftp.readthedocs.io/en/release_0.2.9/pysftp.html?#pysftp.CnOpts
https://pysftp.readthedocs.io/en/release_0.2.9/cookbook.html

**Use case / motivation**

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I want to be able to access SFTP servers that have disabled access from connection with weak ciphers.

airflow/providers/sftp/hooks/sftp.py
"""
        Deletes a directory on the remote system.
        :param path: full path to the remote directory to delete
        :type path: str
        """
        conn = self.get_conn()
        conn.rmdir(path)
    def retrieve_file(self, remote_full_path: str, local_full_path: str) -> None:
        """
        Transfers the remote file to a local location.
        If local_full_path is a string path, the file will be put
        at that location
        :param remote_full_path: full path to the remote file
        :type remote_full_path: str
        :param local_full_path: full path to the local file
        :type local_full_path: str
        """
        conn = self.get_conn()
        self.log.info('Retrieving file from FTP: %s', remote_full_path)
        conn.get(remote_full_path, local_full_path)
        self.log.info('Finished retrieving file from FTP: %s', remote_full_path)
    def store_file(self, remote_full_path: str, local_full_path: str) -> None: