[release/6.0-rc1] Support reference type custom converters in source gen #57954
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Backport of #57592 to release/6.0-rc1
This PR replaces #57772 which only targeted release/6.0.
Fixes #56864
/cc @steveharter
Customer Impact
Broken scenario: custom converters based on reference types result in a compile error when the custom converter is applied to a property via [JsonConverter]; no feasible workaround other than to remove the [JsonConverter] attribute.
Testing
New tests added to verify struct-based and reference-type-based custom converters.
Risk
Low - changes are local to source-gen support of custom converters.