Add handlers for ARC, which can be enabled by the user through the configuration settings.
The handlers if enabled, are added to the BasePipeline. Specifically, the two handlers - authentication_results & sign_message, are added.
The authentication_results handler is responsible for generating the preliminary Authentication Results before the message is altered by any other handlers. And the sign_message handler does the actual signing of the message, i.e. affixing the ARC-Authentication-Results, ARC-Message-Signature, ARC-Seal to the message before it goes out for delivery to the outgoing runner.
A few tests have been added to check the proper functioning of the pipelines with the new handlers. Since some of the tests require the presence of a local key for signing purposes, a test key has been created and stored in a new directory test_data inside the tests directory.
Currently the code for the two new dependencies - arc and dkim (refactored version) is not available on PyPI. As a workaround, os.system explicit calls have been added temporarily to the setup script to install these dependencies from their repository code.