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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
"""Closes the connection"""
        if self.conn is not None:
            self.conn.close()
            self.conn = None
    def describe_directory(self, path: str) -> Dict[str, Dict[str, str]]:
        """
        Returns a dictionary of {filename: {attributes}} for all files
        on the remote system (where the MLSD command is supported).
        :param path: full path to the remote directory
        :type path: str
        """
        conn = self.get_conn()
        flist = conn.listdir_attr(path)
        files = {}
        for f in flist:
            modify = datetime.datetime.fromtimestamp(f.st_mtime).strftime('%Y%m%d%H%M%S')
            files[f.filename] = {
                'size': f.st_size,
                'type': 'dir' if stat.S_ISDIR(f.st_mode) else 'file',
                'modify': modify,
            }
        return files
    def list_directory(self, path: str) -> List[str]: