[invoke/verifyState] Add optional `accountLinkingUrl` return type for invoke

To support a change in the Teams FE, we need to optionally allow developers to configure a static account linking url that, if present, would be returned to the Teams Platform after processing of the activity.

Platform Code:
https://skype.visualstudio.com/SCC/_git/async_messaging_botapiservice/pullrequest/1264762?_a=files#:~:text=var%20obj%20%3D%20JToken,string)obj%3F%5B%22accountLinkingUrl%22%5D%3B

Response Type:
https://github.com/microsoft/teams.net/blob/main/Libraries/Microsoft.Teams.Apps/Response.cs

Sample:

```csharp
new App(new()
{
OAuth: new()
{
AccountLinkingUrl: '...'
}
});
```

Start by examining the invoke/verifyState handling and the Response.cs type linked in the issue, then compare the provided App configuration sample with the existing OAuth response path. Done means an optional accountLinkingUrl can be configured and is returned to the Teams Platform when present, without changing responses when it is absent.

Written by the indexing model from the issue text.