Enhanced slack support

[tracyboehrer]

• Support direct-to-slack responses, allowing full range of slack API requests/content that ABS does not support
• Implemented as a SlackAgentExtension at the moment
• Provides flat models that match slack property naming (snake_case) for easing the use of slack docs vs the models in code.
• Provides path based property retrieval

Example:

    private async Task OnSlackMessageAsync(ITurnContext turnContext, ITurnState turnState, CancellationToken cancellationToken)
    {
        var channelData = turnContext.Activity.GetChannelData<SlackChannelData>();

        var message = new 
        { 
            channel = channelData.EventEnvelope?.Get<string>("event.channel"), 
            text = $"You said: {turnContext.Activity.Text}" 
        };

        await Slack.CallAsync(turnContext, "chat.postMessage", message, channelData.ApiToken, cancellationToken);
    }