Channels class has incomplete list of channel identifiers

[JonathanFingold]

Version

Daily.

Describe the bug

The channels list is incomplete, missing channels visible in Azure: Bing and Skype for Business.

To Reproduce

Steps to reproduce the behavior:

1. Go to https://github.com/Microsoft/botbuilder-dotnet/blob/master/libraries/Microsoft.Bot.Connector/Channels.cs
   • Use a non-MS-internal account.
2. Notice which channels are missing.

Expected behavior

All channels available through Azure have identifiers defined in the Channels class.

• Bonus points for putting these constants in alpha order.

[bug]

[JonathanFingold]

Also, is Bing still an available channel?

[cleemullins]

Note that the Bing channel is no longer supported. Verify this same change in the JS codebase.

[cleemullins]

Fixed in #1328

[cleemullins]

Rather, fixed in #1319