Covariant THub-Parameter for IHubContext

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

Right now, the THub-Paramter of [IHubContext](https://github.com/dotnet/aspnetcore/blob/main/src/SignalR/server/Core/src/IHubContext%60T.cs) isn't covariant what makes the following cast invalid:

```
IHubContext? concrete = null;
IHubContext, IHubClient>? loose = null;

loose = concrete;
```

I need this because I'd like to redirect the HubContext to my concrete's hub context without needing to know the concrete type for consumers:

`serviceCollection.AddSingleton, IHubClient>>(sp => sp.GetRequiredService>());`

This also maybe is a bit related to https://github.com/dotnet/aspnetcore/issues/41538? Is there a reason for not making it covariant? A soon fix would be awesome.

### Describe the solution you'd like

Quite easy: `public interface IHubContext`

### Additional context

_No response_