[PORT] [OAuth] small oauth fix

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3333

The existing overload of GetOauthSignInLinkAsync function with a userId as argument isn't working. It was hardcoding directline, passing empty conversations etc. This PR is to fix that.

Changed projects

• Microsoft.Bot.Builder

[tracyboehrer]

Closing. Python was already populating the ConversationReference per the C# changes.