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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
"""
        Returns a list of files on the remote system.
        :param path: full path to the remote directory to list
        :type path: str
        """
        conn = self.get_conn()
        files = conn.listdir(path)
        return files
    def create_directory(self, path: str, mode: int = 777) -> None:
        """
        Creates a directory on the remote system.
        :param path: full path to the remote directory to create
        :type path: str
        :param mode: int representation of octal mode for directory
        """
        conn = self.get_conn()
        conn.makedirs(path, mode)
    def delete_directory(self, path: str) -> None: