feat: add STATIC, CACHED reasons (#101)

As per:
https://github.com/open-feature/spec/blob/7ed7442c40d4e766424671258846b21a96adda70/specification/types.md#resolution-details

Signed-off-by: Todd Baert <[email protected]>

Author: toddbaert

src/OpenFeature/Constant/Reason.cs

@@ -26,6 +26,16 @@ public static class Reason
         /// </summary>
         public static string Default = "DEFAULT";
 
+        /// <summary>
+        /// The resolved value is static (no dynamic evaluation)
+        /// </summary>
+        public static string Static = "STATIC";
+
+        /// <summary>
+        /// The resolved value was retrieved from cache
+        /// </summary>
+        public static string Cached = "CACHED";
+
         /// <summary>
         /// Use when an unknown reason is encountered when evaluating flag.
         /// An example of this is if the feature provider returns a reason that is not defined in the spec