Add hooks to allow providing additional feature flags in EX pro

We modify the existing `FeaturesProvider` interface so that there can be more
than one of them, and the Features provided by *all* of the instances are
shown. We can then make another instance in element-android-enterprise, which
exposes enterprise-only features.

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

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.featureflag.api
+
+/**
+ * Provides a list of available [Feature]s that the application supports.
+ * Used by [FeatureFlagService.getAvailableFeatures] to obtain a list of features
+ * that can be presented to the user.
+ *
+ * Multiple instances can exist within the application, and all instances are polled
+ * to provide features for the list. This means that additional features can be provided
+ * by compile-time extensions.
+ */
+fun interface FeaturesProvider {
+    fun provide(): List<Feature>
+}

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.FeaturesProvider
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.flowOf
 
@@ -22,7 +23,7 @@ import kotlinx.coroutines.flow.flowOf
 class DefaultFeatureFlagService(
     private val providers: Set<@JvmSuppressWildcards FeatureFlagProvider>,
     private val buildMeta: BuildMeta,
-    private val featuresProvider: FeaturesProvider,
+    private val featuresProviders: Set<FeaturesProvider>,
 ) : FeatureFlagService {
     override fun isFeatureEnabledFlow(feature: Feature): Flow<Boolean> {
         return if (feature.isFinished) {
@@ -47,7 +48,7 @@ class DefaultFeatureFlagService(
         includeFinishedFeatures: Boolean,
         isInLabs: Boolean,
     ): List<Feature> {
-        return featuresProvider.provide().filter { flag ->
+        return featuresProviders.flatMap { provider -> provider.provide() }.filter { flag ->
             (includeFinishedFeatures || !flag.isFinished) &&
                 flag.isInLabs == isInLabs
         }

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

@@ -9,15 +9,12 @@
 package io.element.android.libraries.featureflag.impl
 
 import dev.zacsweers.metro.AppScope
-import dev.zacsweers.metro.ContributesBinding
+import dev.zacsweers.metro.ContributesIntoSet
 import io.element.android.libraries.featureflag.api.Feature
 import io.element.android.libraries.featureflag.api.FeatureFlags
+import io.element.android.libraries.featureflag.api.FeaturesProvider
 
-fun interface FeaturesProvider {
-    fun provide(): List<Feature>
-}
-
-@ContributesBinding(AppScope::class)
+@ContributesIntoSet(AppScope::class)
 class DefaultFeaturesProvider : FeaturesProvider {
     override fun provide(): List<Feature> = FeatureFlags.entries
 }