Add: On creating metadata some constants can be defined (coscine/issues#134)

coscine/issues#134

src/ProxyApi/Utils/PropertyDescription.cs

+using System.Runtime.Serialization;
+
+namespace Coscine.ProxyApi.Utils
+{
+    [DataContract]
+    public class PropertyDescription
+    {
+        [DataMember]
+        public string Value { get; set; }
+        [DataMember]
+        public string Type { get; set; }
+
+        public PropertyDescription(string value, string type)
+        {
+            this.Value = value;
+            this.Type = type;
+        }
+    }
+}