Add Psalm Taint Flow Analysis

This adds the Psalm Security Analysis, as described at
https://psalm.dev/docs/security_analysis/

It also adds a plugin for adding input into AppFramework.

The results can be viewed in the GitHub Security tab at
https://github.com/nextcloud/server/security/code-scanning

**Q&A:**

Q: Why do you not use the shipped Psalm version?
A: I do a lot of changes to the Psalm Taint behaviour. Using released
versions is not gonna get us the results we want.

Q: How do I improve false positives?
A: https://psalm.dev/docs/security_analysis/avoiding_false_positives/

Q: How do I add custom sources?
A: https://psalm.dev/docs/security_analysis/custom_taint_sources/

Q: We should run this on apps!
A: Yes.

Q: What will change in Psalm?
A: Quite some of the PHP core functions are not yet marked to propagate
the taint. This leads to results where the taint flow is lost. That's
something that I am currently working on.

Q: Why is the plugin MIT licensed?
A: Because its the first of its kind (based on GitHub Code Search) and
I want other people to copy it if they want to. Security is for all :)

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>

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