Support OnXXX methods/attributes for System.Text.Json

[StevenRasmussen]

I saw issue #29177 which is currently closed. However, it talks about both converts and the OnXXX methods but only converters has been implemented. I was really hoping this was on the roadmap for completion in the .Net 5 timeframe but don't see a separate ticket for this so I apologize if there is another ticket already open for this.

Writing a custom converter for every type for which you want to be able to provide OnXXX methods is quite a pain. Being able to create a base class with these methods and then being able to override them in inheriting classes has been my pattern which works quite well with Newtonsoft.Json.

[layomia]

We won't get to this in 5.0, but it can be considered in the next release. See the JSON docs for an example workaround for these callbacks.

[layomia]

@steveharter is this issue a duplicate of #54528?

[steveharter]

@steveharter is this issue a duplicate of #54528?

Yes. Since this issue didn't propose an API I created the API issue instead.