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>
+}