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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.path_exists(path)
        self.assertEqual(result, exists)
    @parameterized.expand(
        [
            ("test/path/file.bin", None, None, True),
            ("test/path/file.bin", "test", None, True),
            ("test/path/file.bin", "test/", None, True),
            ("test/path/file.bin", None, "bin", True),
            ("test/path/file.bin", "test", "bin", True),
            ("test/path/file.bin", "test/", "file.bin", True),
            ("test/path/file.bin", None, "file.bin", True),
            ("test/path/file.bin", "diff", None, False),
            ("test/path/file.bin", "test//", None, False),
            ("test/path/file.bin", None, ".txt", False),
            ("test/path/file.bin", "diff", ".txt", False),
        ]
    )
    def test_path_match(self, path, prefix, delimiter, match):