[PORT] BotAdapter changes (made for custom adapter / skills compat)

[tomlm]

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

#3147 - Added additional ContinueConversationAsync overload for custom adapters for skills compatibility.
#3146 - Move 'BotIdentityKey' const to BotAdapter and made it public.

[garypretty]

Custom adapters do not currently exist for Python - just requires move 'BotIdentityKey' const to BotAdapter and made it public for parity.