[PORT] Event factory for handoff protocol

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3347

From https://github.com/microsoft/BotBuilder-Samples/tree/master/experimental/handoff-library with minor changes

Changed projects

• Microsoft.Bot.Builder
• Microsoft.Bot.Builder.Tests

[Stevenic]

Needs porting

[stevengum]

This is in progress, following up with artul and carlosscastro offline.