fix: Adding Async Lifetime method to fix flaky unit tests (#333)

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## This PR
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- Changes the way we currently reset the Api between unit tests. This
approach should be safer since it calls the official `Shutdown`, and
when we call the API again, all the resources are reset.

### Notes
<!-- any additional notes for this PR -->
Check for more details:
https://xunit.net/docs/shared-context#:~:text=For%20context%20cleanup%2C%20add%20the%20IDisposable%20interface%20to,call%20IAsyncDisposable%20%28it%20is%20planned%20for%20xUnit%20v3%29.

---------

Signed-off-by: André Silva <2493377+askpt@users.noreply.github.com>

src/OpenFeature/Api.cs

@@ -29,7 +29,7 @@ public sealed class Api : IEventBus
         /// <summary>
         /// Singleton instance of Api
         /// </summary>
-        public static Api Instance { get; } = new Api();
+        public static Api Instance { get; private set; } = new Api();
 
         // Explicit static constructor to tell C# compiler
         // not to mark type as beforeFieldInit
@@ -300,5 +300,13 @@ private async Task AfterError(FeatureProvider provider, Exception? ex)
 
             await this._eventExecutor.EventChannel.Writer.WriteAsync(new Event { Provider = provider, EventPayload = eventPayload }).ConfigureAwait(false);
         }
+
+        /// <summary>
+        /// This method should only be using for testing purposes. It will reset the singleton instance of the API.
+        /// </summary>
+        internal static void ResetApi()
+        {
+            Instance = new Api();
+        }
     }
 }

test/OpenFeature.Tests/ClearOpenFeatureInstanceFixture.cs

@@ -1,14 +1,20 @@
-using System;
+using System.Threading.Tasks;
+using Xunit;
 
 namespace OpenFeature.Tests;
 
-public class ClearOpenFeatureInstanceFixture : IDisposable
+public class ClearOpenFeatureInstanceFixture : IAsyncLifetime
 {
+    public Task InitializeAsync()
+    {
+        Api.ResetApi();
+
+        return Task.CompletedTask;
+    }
+
     // Make sure the singleton is cleared between tests
-    public void Dispose()
+    public async Task DisposeAsync()
     {
-        Api.Instance.SetContext(null);
-        Api.Instance.ClearHooks();
-        Api.Instance.SetProviderAsync(new NoOpFeatureProvider()).Wait();
+        await Api.Instance.ShutdownAsync().ConfigureAwait(false);
     }
 }