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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
result = self.hook._is_path_match(path=path, prefix=prefix, delimiter=delimiter)
        self.assertEqual(result, match)
    def test_get_tree_map(self):
        tree_map = self.hook.get_tree_map(path=os.path.join(TMP_PATH, TMP_DIR_FOR_TESTS))
        files, dirs, unknowns = tree_map
        self.assertEqual(files, [os.path.join(TMP_PATH, TMP_DIR_FOR_TESTS, SUB_DIR, TMP_FILE_FOR_TESTS)])
        self.assertEqual(dirs, [os.path.join(TMP_PATH, TMP_DIR_FOR_TESTS, SUB_DIR)])
        self.assertEqual(unknowns, [])
    def tearDown(self):
        shutil.rmtree(os.path.join(TMP_PATH, TMP_DIR_FOR_TESTS))
        os.remove(os.path.join(TMP_PATH, TMP_FILE_FOR_TESTS))
        self.update_connection(self.old_login)