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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
kwargs['ssh_conn_id'] = ftp_conn_id
        super().__init__(*args, **kwargs)
        self.conn = None
        self.private_key_pass = None
        
        self.no_host_key_check = False
        if self.ssh_conn_id is not None:
            conn = self.get_connection(self.ssh_conn_id)
            if conn.extra is not None:
                extra_options = conn.extra_dejson
                if 'private_key_pass' in extra_options:
                    self.private_key_pass = extra_options.get('private_key_pass')
                
                
                import warnings
                if 'ignore_hostkey_verification' in extra_options:
                    warnings.warn(
                        'Extra option `ignore_hostkey_verification` is deprecated.'
                        'Please use `no_host_key_check` instead.'
                        'This option will be removed in Airflow 2.1',
                        DeprecationWarning,
                        stacklevel=2,
                    )
                    self.no_host_key_check = (
                        str(extra_options['ignore_hostkey_verification']).lower() == 'true'
                    )
                if 'no_host_key_check' in extra_options:
                    self.no_host_key_check = str(extra_options['no_host_key_check']).lower() == 'true'
                if 'private_key' in extra_options: