docs: add encryption operator

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Plugin Engine

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For details on using the Plugin Engine, see the :ref:`Plugins` documentation.

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Encryption

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The Encryption Operator uses the :ref:`Plugins` interface.

This operator uses `libsodium <https://doc.libsodium.org/>`_ for encrypting and decrypting data.

If ADIOS can find libsodium at configure time, this plugin will be built.

This operator will generate a secret key and encrypts the data with the key and a nonce as described in the libsodium `secret key cryptography docs <https://doc.libsodium.org/secret-key_cryptography/secretbox>`_.

The key is saved to the specified ``SecretKeyFile`` and will be used for decryption. The key should be kept confidential since it is used to both encrypt and decrypt the data.

Parameters to use with the Encryption operator:

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 **Key**                       **Value Format**       **Explanation**

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 PluginName                     string                Required. Name to refer to plugin, e.g., ``MyOperator``

 PluginLibrary                  string                Required. Name of shared library, ``EncryptionOperator``

 SecretKeyFile                  string                Required. Path to secret key file

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2. :ref:`Runtime Configuration Files` in the :ref:`ADIOS` component.

.. include:: ZFPCompressor.rst

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Plugin Operator

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For details on using the Plugin Operator, see the :ref:`Plugins` documentation.