Add InvokeReferences support to InvokeAsync method

Added InvokeReferences class for passing optional reference values (Reference1–Reference5) when invoking a profile. Updated InvokeAsync to accept and serialize these references in the POST request. Improved XML docs to reflect the new parameter.

src/ReC.Client/Api/InvokeReferences.cs

@@ -0,0 +1,46 @@
+namespace ReC.Client.Api
+{
+    /// <summary>
+    /// Optional reference values that are passed through to all result records when invoking a profile.
+    /// </summary>
+    public class InvokeReferences
+    {
+        /// <summary>Reference value 1.</summary>
+        public string
+#if NET
+            ?
+#endif
+            Reference1 { get; set; }
+
+        /// <summary>Reference value 2.</summary>
+        public string
+#if NET
+            ?
+#endif
+            Reference2 { get; set; }
+
+        /// <summary>Reference value 3.</summary>
+        public string
+
+#if NET
+            ?
+#endif
+            Reference3 { get; set; }
+
+        /// <summary>Reference value 4.</summary>
+        public string
+
+#if NET
+            ?
+#endif
+            Reference4 { get; set; }
+
+        /// <summary>Reference value 5.</summary>
+        public string
+
+#if NET
+            ?
+#endif
+            Reference5 { get; set; }
+    }
+}