Support JsonExtensionData in source generator fast path

[layomia]

Per #51945 (comment) and #58912 (comment), the feature is currently not supported in fast-path serialization and the serializer falls back to a metadata-based implementation if provided/configured by the source generator.

We should target 7.0 - it is technically feasible to do this now, just a bit complex & a large diff at this stage in 6.0. Concretely, extension data properties (dictionaries) can have typeof(object) values which is a polymorphic scenario currently not supported in fast-path.

We should also consider deserialization support for this feature, alongside #55043 ("Consider fast-path deserialization logic in JSON source generator").

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Issue Details

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Per #51945 (comment) and #58912 (comment), the feature is currently not supported and the serializer falls back to a metadata-based implementation if provided/configured by the source generator.

We should target 7.0 - it is technically feasible to do this now, just a bit complex & a large diff at this stage in 6.0. Concretely, extension data properties (dictionaries) can have typeof(object) values which is a polymorphic scenario currently not supported in fast-path.

Author:	layomia
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	7.0.0

[eiriktsarpalis]

@layomia should his be included in the planned work epic?