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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
connection = Connection(login='login', host='host', extra='{"no_host_key_check": "foo"}')
        get_connection.return_value = connection
        hook = SFTPHook()
        self.assertEqual(hook.no_host_key_check, False)
    @mock.patch('airflow.providers.sftp.hooks.sftp.SFTPHook.get_connection')
    def test_no_host_key_check_ignore(self, get_connection):
        connection = Connection(login='login', host='host', extra='{"ignore_hostkey_verification": true}')
        get_connection.return_value = connection
        hook = SFTPHook()
        self.assertEqual(hook.no_host_key_check, True)
    @mock.patch('airflow.providers.sftp.hooks.sftp.SFTPHook.get_connection')
    def test_no_host_key_check_no_ignore(self, get_connection):
        connection = Connection(login='login', host='host', extra='{"ignore_hostkey_verification": false}')
        get_connection.return_value = connection
        hook = SFTPHook()
        self.assertEqual(hook.no_host_key_check, False)
    @parameterized.expand(
        [
            (os.path.join(TMP_PATH, TMP_DIR_FOR_TESTS), True),
            (os.path.join(TMP_PATH, TMP_FILE_FOR_TESTS), True),
            (os.path.join(TMP_PATH, TMP_DIR_FOR_TESTS + "abc"), False),
            (os.path.join(TMP_PATH, TMP_DIR_FOR_TESTS, "abc"), False),
        ]
    )
    def test_path_exists(self, path, exists):