[API Proposal]: JsonDerivedTypeAttribute<T> for simplified polymorphic JSON configuration

[kzu]

Background and motivation

You currently have to decide on a "discriminator name" for derived types, such as:

[JsonDerivedType(typeof(MessageContent), nameof(MessageContent)]
[JsonDerivedType(typeof(FileContent), nameof(FileContent)]
public abstract record Content ...

In many cases, choosing the type name as the discrimantor makes perfect sense.
Since attributes can now be generic, it would make sense to leverage that to simplify the annotation.

API Proposal

public class JsonDerivedTypeAttribute<T> : JsonDerivedTypeAttribute
{
    public JsonDerivedTypeAttribute(string? typeDiscriminator = default) : base(typeof(T), typeDiscriminator ?? typeof(T).Name)
    {
    }
}```


### API Usage

```csharp
[JsonDerivedType<MessageContent>]
[JsonDerivedType<FileContent>("file")]
public abstract record Content ...

Alternative Designs

Perhaps the overload to override the discriminator string is unnecessary?

Risks

No response

[eiriktsarpalis]

Duplicate of #72936.