system.text.json support for polymorphic serialization.

[qsdfplkj]

There has been questions before about polymorphic (de)serialization of json. There is an argument to not support this because it opens a security vulnerability where completely unrelated classes could be instantiated and allow execute of arbitrary code. But why not allow for custom interfaces or types to be used. Just Deserialize<IMyInterface> shouldn't be causing problems?

[Symbai]

Dupe of #29937 and #30083, please vote on both and comment on them if you support the same request.

[layomia]

Closing as dup of issues @Symbai linked above.