[release/6.0] Support JsonExtensionData in STJ source-gen

[github-actions]

Backport of #58912 to release/6.0

Fixes #58273 for 6.0.

/cc @eiriktsarpalis @layomia

Customer Impact

Fixes incomplete (major) scenario for new 6.0 source-gen feature: JsonExtensionData support.

Testing

Tests have been shared to ensure functional parity between src-gen and reflection serializers.

Risk

Low risk to existing reflection-based serializer (i.e. no changes to tests and limited changes to src/ implementation). Low risk for src-gen as well - it is new and shares same tests with pre-existing serializer.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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Issue Details

---

Backport of #58912 to release/6.0

/cc @eiriktsarpalis @layomia

Customer Impact

Testing

Risk

Author:	github-actions[bot]
Assignees:	-
Labels:	area-System.Text.Json, new-api-needs-documentation
Milestone:	-

[eiriktsarpalis]

We need to backport #58993 after this is merged.

[danmoseley]

Approved. JsonExtensionData is fairly widely used and does not work with this new source generator feature. Churn is largely limited to the generator.

[danmoseley]

I'm going to merge this because there is no sign of CI improving before the branch, the possibility of OS-specific bugs in the blocked queues in this change is very small, and other PR's are blocked on this.