[PORT] make custom function easier

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3595

Make customized function in LG and expression easier by accepting just a lambda.

At the cost of expose FunctionTable instead of a generic IDictionary.

Now you can do

            Expression.Functions.Add("contoso.sqrt", (args) =>
            {
                object retValue = null;
                if (args[0] != null)
                {
                    double dblValue;
                    if (double.TryParse(args[0], out dblValue))
                    {
                        retValue = Math.Sqrt(dblValue);
                    }
                }

                return retValue;
            });

to add a new function

Changed projects

• AdaptiveExpressions
• Microsoft.Bot.Builder.LanguageGeneration.Tests