refactor: Provider enhancements (#43)

refactoring on provider

- add `readonly` modifier if possible
- sealed private class `StatusProvider`

Author: WeihanLi

src/LaunchDarkly.OpenFeature.ServerProvider/Provider.StatusProvider.cs

@@ -8,14 +8,14 @@ namespace LaunchDarkly.OpenFeature.ServerProvider
 {
     public sealed partial class Provider
     {
-        private class StatusProvider
+        private sealed class StatusProvider
         {
             private ProviderStatus _providerStatus = ProviderStatus.NotReady;
             private bool _firstEvent = true;
-            private object _statusLock = new object();
-            private Channel<object> _eventChannel;
-            private string _providerName;
-            private Logger _logger;
+            private readonly object _statusLock = new object();
+            private readonly Channel<object> _eventChannel;
+            private readonly string _providerName;
+            private readonly Logger _logger;
 
             public StatusProvider(Channel<object> eventChannel, string providerName, Logger logger)
             {