[PORT] Add 'ephemeral' as the list of options for Delivery Mode

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3502

We are adding a new option for DeliveryMode on the Activity. This property is a string, and the code comment lists all of the possible values. I have updated the code comment to list the new value of 'ephemeral'.

Changed projects

• Microsoft.Bot.Schema