[PM-1011] LaunchDarkly service (#2726)

* LaunchDarkly service

* Load file-based flag values, properly support offline only when self-host

* Simplify tests

* Use interface for LD settings

* Remove tests that will provide inconsistent results depending on machine setup and file fallback

* Fall back to offline mode more actively

* Drive fallback file path with setting

src/Core/Services/IFeatureService.cs

@@ -0,0 +1,6 @@
+namespace Bit.Core.Services;
+
+public interface IFeatureService
+{
+    bool IsOnline();
+}

src/Core/Services/Implementations/LaunchDarklyFeatureService.cs

@@ -0,0 +1,54 @@
+using Bit.Core.Settings;
+using LaunchDarkly.Sdk.Server;
+using LaunchDarkly.Sdk.Server.Integrations;
+
+namespace Bit.Core.Services;
+
+public class LaunchDarklyFeatureService : IFeatureService, IDisposable
+{
+    private readonly LdClient _client;
+
+    public LaunchDarklyFeatureService(
+        IGlobalSettings globalSettings)
+    {
+        var ldConfig = Configuration.Builder(globalSettings.LaunchDarkly?.SdkKey);
+
+        if (string.IsNullOrEmpty(globalSettings.LaunchDarkly?.SdkKey))
+        {
+            // support a file to load flag values
+            if (File.Exists(globalSettings.LaunchDarkly?.FlagDataFilePath))
+            {
+                ldConfig.DataSource(
+                    FileData.DataSource()
+                        .FilePaths(globalSettings.LaunchDarkly?.FlagDataFilePath)
+                        .AutoUpdate(true)
+                );
+
+                // do not provide analytics events
+                ldConfig.Events(Components.NoEvents);
+            }
+            else
+            {
+                // when a file-based fallback isn't available, work offline
+                ldConfig.Offline(true);
+            }
+        }
+        else if (globalSettings.SelfHosted)
+        {
+            // when self-hosted, work offline
+            ldConfig.Offline(true);
+        }
+
+        _client = new LdClient(ldConfig.Build());
+    }
+
+    public bool IsOnline()
+    {
+        return _client.Initialized && !_client.IsOffline();
+    }
+
+    public void Dispose()
+    {
+        _client?.Dispose();
+    }
+}