Introduce SAML SP-initiated Logout to SATOSA proxy

[sebulibah]

This PR introduces SAML Single Logout support for SATOSA. With this addition users will be able to log out of SAML Service Providers, improving security and user experience.

• Detailed description
• Slides

Changes

• Register single_logout_service methods to endpoints
• Register logout_callback functions
• Add functions to handle logout requests and logout responses
• Add a database to store session information required for logout

Testing

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• [ ] Have you written new tests for your changes?
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