Scaffolded the CompositeRuntime key

Author: darrelmiller

src/Microsoft.OpenApi/Expressions/CompositeExpression.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Microsoft.OpenApi.Expressions
+{
+    public class CompositeExpression : RuntimeExpression
+    {
+        private string template;
+
+        public CompositeExpression(string expression)
+        {
+            template = expression;
+
+            // Extract subexpressions and convert to RuntimeExpressions
+
+        }
+
+        public override string Expression => template;
+    }
+}

src/Microsoft.OpenApi/Expressions/RuntimeExpression.cs

@@ -34,6 +34,11 @@ public static RuntimeExpression Build(string expression)
                 throw Error.ArgumentNullOrWhiteSpace(nameof(expression));
             }
 
+            if (expression.Contains("{$"))
+            {
+                return new CompositeExpression(expression);
+            }
+
             if (!expression.StartsWith(Prefix))
             {
                 throw new OpenApiException(string.Format(SRResource.RuntimeExpressionMustBeginWithDollar, expression));