[PORT] Update adapters for Skills compatibility

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3148

microsoft/botbuilder-dotnet#3147 - Added additional ContinueConversationAsync overload for custom adapters for skills compatibility.
microsoft/botbuilder-dotnet#3146 - Move 'BotIdentityKey' const to BotAdapter and made it public. Done via #1763

[Stevenic]

These changes need to be applied to all JS adapters for skills to work.

[benbrown]

Part 1 is here -> #1763, this includes moving the BotIdentityKey to BotAdapter and changing the BotFrameworkAdapter's implementation of continueConversation to include the overload.

The other change will need to be applied to the adapters hosted in the Botkit repo... which will have to happen after the changes to BotAdapter are published.

[stevengum]

We moved the BotIdentityKey to the BotAdapter in #1736, so porting microsoft/botbuilder-dotnet#3146 has been completed.

[cleemullins]

Closing per Ben and Steven.