Add NETSTANDARD specific method for getting config setting with provenance.

Author: tippmar-nr

src/Agent/NewRelic/Agent/Core/Config/ConfigurationLoader.cs

@@ -111,6 +111,14 @@ public static ValueWithProvenance<string> GetConfigSetting(string key)
 				value = new ValueWithProvenance<string>(ConfigurationManager.AppSettings[key],
 					"ConfigurationManager app setting");
 			}
+#else
+            if (value?.Value == null)
+            {
+                var configMgrStatic = new ConfigurationManagerStatic();
+                var configValue = configMgrStatic.GetAppSetting(key);
+                if (configValue != null)
+                    value = new ValueWithProvenance<string>(configValue, configMgrStatic.AppSettingsFilePath);
+            }
 #endif
             return value;
         }