.NET 6/7 Support for WriteFormPost

**Describe the bug**
In .NET Framework we have something like the following code:
```
...
RequestSecurityTokenResponse rstr = null;
channel.Issue(rst, out rstr)
var signIn = new SignInResponseMessage(uri, rstr, new WSFederationSerializer(), new WSTrustSerializationContext());
var formPost = signIn.WriteFormPost();
...
```

Would this be possible to do with the currently released nuget packages in .NET 6/7? If so, how? It starts with the missing out parameter for Issue/IssueAsync and I haven't found another way since the needed classes is private/internal.

**Expected behavior**
To be able to get the response as a form post that can be written to a web browser control in a desktop client.

@mconnew helped me a lot during spring. I guess you might any idea about this as well? 😄