Prevent usage of ':'

Author: jimmyca15

src/Microsoft.FeatureManagement/ConfigurationFeatureDefinitionProvider.cs

@@ -47,6 +47,11 @@ public Task<FeatureDefinition> GetFeatureDefinitionAsync(string featureName)
                 throw new ArgumentNullException(nameof(featureName));
             }
 
+            if (featureName.Contains(ConfigurationPath.KeyDelimiter))
+            {
+                throw new ArgumentException($"The value '{ConfigurationPath.KeyDelimiter}' is not allowed in the feature name.", nameof(featureName));
+            }
+
             if (Interlocked.Exchange(ref _stale, 0) != 0)
             {
                 _definitions.Clear();