C#: Add source models for `file` threat model/source kind for .NET standard library

[egregius313]

trafficstars

WIP, but adds source and summary models related to file (streams) in System.IO.

[github-actions[bot]]

:warning: The head of this PR and the base branch were compared for differences in the framework coverage reports. The generated reports are available in the artifacts of this workflow run. The differences will be picked up by the nightly job after the PR gets merged.

Click to show differences in coverage

csharp

Generated file changes for csharp

• Changes to framework-coverage-csharp.rst:

-    System,"``System.*``, ``System``",30,11864,67,9
+    System,"``System.*``, ``System``",44,11872,67,9
-    Totals,,42,13418,409,9
+    Totals,,56,13426,409,9

• Changes to framework-coverage-csharp.csv:

- System,67,30,11864,,8,8,9,,,4,5,,33,2,3,1,17,3,4,,9898,1966
+ System,67,44,11872,,8,8,9,,,4,5,,33,2,3,15,17,3,4,,9906,1966

[michaelnebel]

This PR needs to be re-based and the flow summaries test needs to be updated (due to https://github.com/github/codeql/pull/15940)

[michaelnebel]

DCA looks good. We find 30+ more results with the new sources and summaries and it doesn't appear to have any impact on performance.