Refactor BotFrameworkAdapter Auth flow

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3409

Fixes: https://github.com/microsoft/botbuilder-dotnet/issues/3386
Addresses microsoft/botframework-sdk#5751 in C#

PR will address doing proactive-messaging for non-channel recipients and caching keys for BotFrameworkAdapter.

PR is in draft because the former item is addressed by current commits.

commit 1385c8f

• add BotFrameworkAdapter.ContinueSkillConversationAsync
• add SkillConversationReference
• make cancellationToken optional in CreateConnectorClientAsync

Changed projects

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

[Skills,WIP]