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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.

tests/providers/sftp/hooks/test_sftp.py
@provide_session
    def update_connection(self, login, session=None):
        connection = session.query(Connection).filter(Connection.conn_id == "sftp_default").first()
        old_login = connection.login
        connection.login = login
        session.commit()
        return old_login
    def setUp(self):
        self.old_login = self.update_connection(SFTP_CONNECTION_USER)
        self.hook = SFTPHook()
        os.makedirs(os.path.join(TMP_PATH, TMP_DIR_FOR_TESTS, SUB_DIR))
        with open(os.path.join(TMP_PATH, TMP_FILE_FOR_TESTS), 'a') as file:
            file.write('Test file')
        with open(os.path.join(TMP_PATH, TMP_DIR_FOR_TESTS, SUB_DIR, TMP_FILE_FOR_TESTS), 'a') as file:
            file.write('Test file')
    def test_get_conn(self):
        output = self.hook.get_conn()
        self.assertEqual(type(output), pysftp.Connection)
    def test_close_conn(self):
        self.hook.conn = self.hook.get_conn()
        self.assertTrue(self.hook.conn is not None)
        self.hook.close_conn()
        self.assertTrue(self.hook.conn is None)
    def test_describe_directory(self):