Add a feature flag to reuse the last pos value for initial syncs (#5010)

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlags.kt

@@ -175,4 +175,13 @@ enum class FeatureFlags(
         // False so it's displayed in the developer options screen
         isFinished = false,
     ),
+    SharePos(
+        key = "feature.share_pos",
+        title = "Share pos in sliding sync",
+        description = "Keep the sliding sync pos to make initial syncs faster. Requires an app restart to take effect." +
+            "\n\nWARNING: this may cause issues with syncs.",
+        defaultValue = { false },
+        // False so it's displayed in the developer options screen
+        isFinished = false,
+    ),
 }