feature: add middleware to remove @mentions for MSTeams

[Stevenic]

Is your feature request related to a problem? Please describe.

Messages from MS Teams include an @mention of the bots name at the beginning of every message. This generally needs to be stripped off using ActivityExtensions.RemoveRecipientMention() otherwise TextInput actions will include it as part of the users response. I ejected my host and customized my OnTurnAsync() implementation to include the block below but we should really formalize this into a piece of middleware that can be turned on & off as a feature like the inspection middleware.

       public override async Task OnTurnAsync(ITurnContext turnContext, CancellationToken cancellationToken = default(CancellationToken))
        {
            if (turnContext.Activity.Type == "message") {
                turnContext.Activity.RemoveRecipientMention();
            }

[jwiersem]

I have this same problem when I use botframework composer for creating a MS teams groupchat scoped bot. How do I fix this like you have done @Stevenic ? Or is there a permanent solution for this issue already?