[PORT] [Teams] adding helper to send messages to channel

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3292

Changed projects

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

[R8,teams]

[cleemullins]

@axelsrz This has been sitting since Jan. Can you look, and see if you can take the change and close this out? The C# code seems a very minor change.

[axelsrz]

After I looked at the code and had a conversation with @Virtual-Josh we realized this work has already been done in #653 and #654 so I'm closing this