feat: expose the FDv2 data system through the Flutter SDK

Surface the FDv2 data acquisition protocol on the public Flutter API:
- LDConfig gains a dataSystem option (opts into FDv2; early access).
- LDClient.setConnectionMode([ConnectionMode?]) applies a manual
  connection-mode override (sticks and suppresses automatic transitions;
  null clears it).
- Export the data-system configuration types (DataSystemConfig,
  ConnectionModeId, ModeDefinition, EndpointConfig, the initializer /
  synchronizer entries, and Fdv1FallbackConfig) and drop the internal
  FDv2* connection-mode types from the public surface.

The data system itself is built in common_client; this is the Flutter
wiring that lets applications configure and use it.

Author: kinyoklion

packages/flutter_client_sdk/lib/launchdarkly_flutter_client_sdk.dart

@@ -45,11 +45,6 @@ export 'package:launchdarkly_common_client/launchdarkly_common_client.dart'
         PersistenceConfig,
         ApplicationInfo,
         ConnectionMode,
-        FDv2ConnectionMode,
-        FDv2Streaming,
-        FDv2Polling,
-        FDv2Offline,
-        FDv2Background,
         ResolvedConnectionMode,
         ResolvedStreaming,
         ResolvedPolling,
@@ -73,7 +68,19 @@ export 'package:launchdarkly_common_client/launchdarkly_common_client.dart'
         PluginCredentialInfo,
         PluginEnvironmentMetadata,
         PluginMetadata,
-        PollingConfig;
+        PollingConfig,
+        DataSystemConfig,
+        ConnectionModeId,
+        ModeDefinition,
+        EndpointConfig,
+        InitializerEntry,
+        SynchronizerEntry,
+        CacheInitializer,
+        PollingInitializer,
+        StreamingInitializer,
+        PollingSynchronizer,
+        StreamingSynchronizer,
+        Fdv1FallbackConfig;
 
 export 'src/ld_client.dart' show LDClient;
 export 'src/config/ld_config.dart' show LDConfig, ApplicationEvents;

packages/flutter_client_sdk/lib/src/config/ld_config.dart

@@ -90,6 +90,10 @@ final class LDConfig extends LDCommonConfig {
   /// [plugins] can be used to add plugins to the SDK.
   ///
   /// Plugin support is currently experimental and subject to change.
+  ///
+  /// [dataSystem] opts the SDK into the FDv2 data acquisition protocol.
+  /// This feature is not stable, and not subject to any backwards
+  /// compatibility guarantees or semantic versioning. It is in early access.
   LDConfig(super.sdkCredential, super.autoEnvAttributes,
       {super.applicationInfo,
       super.httpProperties,
@@ -103,7 +107,8 @@ final class LDConfig extends LDCommonConfig {
       super.globalPrivateAttributes,
       ApplicationEvents? applicationEvents,
       super.hooks,
-      List<Plugin>? plugins})
+      List<Plugin>? plugins,
+      super.dataSystem})
       : applicationEvents = applicationEvents ?? ApplicationEvents(),
         plugins =
             plugins != null ? UnmodifiableListView(List.from(plugins)) : null;

packages/flutter_client_sdk/lib/src/ld_client.dart

@@ -343,6 +343,27 @@ interface class LDClient {
     _connectionManager.offline = offline;
   }
 
+  /// Set the connection mode the SDK uses for data acquisition.
+  ///
+  /// The mode is applied as a manual override: it sticks and suppresses
+  /// automatic state-detection transitions (backgrounding, network
+  /// availability) until cleared. Call with no argument (or null) to clear
+  /// the override and resume automatic mode resolution.
+  ///
+  /// This method is not stable, and not subject to any backwards
+  /// compatibility guarantees or semantic versioning. It is in early
+  /// access. If you want access to this feature please join the EAP.
+  /// https://launchdarkly.com/docs/sdk/features/data-saving-mode
+  void setConnectionMode([ConnectionMode? mode]) {
+    _connectionManager.setMode(mode == null
+        ? null
+        : switch (mode) {
+            ConnectionMode.streaming => const FDv2Streaming(),
+            ConnectionMode.polling => const FDv2Polling(),
+            ConnectionMode.offline => const FDv2Offline(),
+          });
+  }
+
   /// Check if the SDK has finished initialization.
   ///
   /// This does not indicate that initialization was successful, but that it is