Add Labs screen for beta testing of public features (#5465)

* Add Labs screen:

- Make `Feature` have an `isInLabs` boolean to distinguish private feature flags from public ones.
- Have `FeatureFlagsService` provide the list of available flags.
- Display the labs item in the settings screen only if there are available public features.
- Remove public feature toggles from developer options.
- Implement the labs screen with the public features.
- Add a clear cache step to the threads feature toggle
- Update screenshots

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Co-authored-by: ElementBot <android@element.io>

libraries/featureflag/impl/src/main/kotlin/io/element/android/libraries/featureflag/impl/DefaultFeatureFlagService.kt

@@ -14,6 +14,7 @@ import dev.zacsweers.metro.SingleIn
 import io.element.android.libraries.core.meta.BuildMeta
 import io.element.android.libraries.featureflag.api.Feature
 import io.element.android.libraries.featureflag.api.FeatureFlagService
+import io.element.android.libraries.featureflag.api.FeatureFlags
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.flowOf
 
@@ -40,4 +41,8 @@ class DefaultFeatureFlagService(
             ?.let { true }
             ?: false
     }
+
+    override fun getAvailableFeatures(): List<Feature> {
+        return FeatureFlags.entries.filter { !it.isFinished }
+    }
 }