Switch to using composer and patches to manage simplesamlphp depedency

[jay-oswald]

Working for me. Unsure about the test fails, seams like every PR is failing tests, so don't think it is set up properly for it?

Do NOT click on files changed tab, it just crashes my entire laptop.

Why?
Current method involves deleting simplesaml2, downloading a new copy, cleaning it up. Finding our customisations, cherry-picking them etc, its a lengthy and error-prone process. With this change we can just run composer update and let it update.
Moodle itself is also moving towards composer native support in 5.1. Once that is out I am hoping our 5.1 branch can just stop committing our vendor folder and have it work with the Moodle vendor system, but that is still TBC
We also now have a patches/ folder with all our customisation's, making it obvious what changes we have made.

Changes

• Remove .extlib with our customised version of simplesamlphp
• Add composer.json to require simplesamlphp
• Commit vendor folder, so composer is not required for usage, still just installs and works same as with .extlib folder
• Set up composer patches, to store our customisations to simplesamlphp
• Set up Dockerfile and docker-compose.yaml NOT for production usage, but as a way to run composer in a standard, repeatable way