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

All Submissions:

• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] Have you added an explanation of what problem you are trying to solve with this PR?
• [x] Have you added information on what your changes do and why you chose this as your solution?
• [ ] Have you written new tests for your changes?
• [ ] Does your submission pass tests?
• [x] This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[niheconomoum]

We presented this work at the 2023 Internet2 Technology Exchange.

[sbliven]

This looks pretty far along. What's blocking the merge?