[PORT] [Expression]Add string interpolation

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#2968

fix: #2785

Changed projects

• Microsoft.Bot.Builder.LanguageGeneration
• Microsoft.Bot.Expressions
• Microsoft.Bot.Builder.LanguageGeneration.Tests
• Microsoft.Bot.Expressions.Tests

[adaptive]

[Danieladu]

combine with #1703