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

@@ -36,4 +36,10 @@ interface Feature {
      * If true: the feature is finished, it will not appear in the developer options screen.
      */
     val isFinished: Boolean
+
+    /**
+     * Whether the feature is only available in Labs (and not in developer options).
+     * Feature flags that set this to `true` can be enabled by any users, not only those that have enabled developer mode.
+     */
+    val isInLabs: Boolean
 }

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

@@ -33,4 +33,9 @@ interface FeatureFlagService {
      * is registered
      */
     suspend fun setFeatureEnabled(feature: Feature, enabled: Boolean): Boolean
+
+    /**
+     * @return the list of available (not finished) features that can be toggled.
+     */
+    fun getAvailableFeatures(): List<Feature>
 }

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

@@ -19,6 +19,7 @@ enum class FeatureFlags(
     override val description: String? = null,
     override val defaultValue: (BuildMeta) -> Boolean,
     override val isFinished: Boolean,
+    override val isInLabs: Boolean = false,
 ) : Feature {
     RoomDirectorySearch(
         key = "feature.roomdirectorysearch",
@@ -98,6 +99,7 @@ enum class FeatureFlags(
         description = "Renders thread messages as a dedicated timeline. Restarting the app is required for this setting to fully take effect.",
         defaultValue = { false },
         isFinished = false,
+        isInLabs = true,
     ),
     MultiAccount(
         key = "feature.multi_account",