Epic: Deliver on developers' top-requested System.Text.Json features and improvements

[cathysull]

There are about 200 open issues for JSON and community engagement continues to be high along with direct asks from authors like NSwag. There are still have some missing features that prevents System.Text.Json to be successful as the standard JSON stack for .NET.

User Stories

• Developers can convert between JSON data and C# dynamic, and also have a mutable JSON DOM type Developers can convert between JSON data and C# dynamic, and also use a mutable JSON DOM type #45188
• Developers using JsonSerializer can handle common F# types well User Story: Developers using JsonSerializer want it to handle common F# types well #29812
• Developers using JsonSerializer can asynchronously (de)serialize IAsyncEnumerable Developers using JsonSerializer can asynchronously (de)serialize IAsyncEnumerable<T> #1570 -- needed by ASP.NET
• Developers using System.Text.Json have their top requests addressed so that they can use STJ in more scenarios Developers using System.Text.Json have their top requests addressed so that they can use STJ in more scenarios #45190
• Developers using JsonSerializer have full polymorphic support Developers using JsonSerializer have support for polymorphic types #45189

[danmoseley]

@ericstj @layomia as you determine the prioritized list of work for JSON for 6.0 please extend the list above so it links up in https://themesof.net/. It would be great to add a Cost label to each.

[danmoseley]

@terrajobst proposes that the headings above (Eg Extend polymorphic support) be User STories themselves, so they can group their child issues in the tree.