Implement a hook allowing enterprise modules to perform actions on startup

Add an interface `io.element.android.features.enterprise.api.AppStartupHook`,
which can be implemented in the enterprise build to perform certain actions on
startup.

It's a bit of a kludge for now since it doesn't block the rest of the app from
proceeding, but it will serve our purposes for now.

features/enterprise/api/src/main/kotlin/io/element/android/features/enterprise/api/AppStartupHook.kt

@@ -0,0 +1,20 @@
+/*
+ * 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.features.enterprise.api
+
+import android.app.Activity
+
+/**
+ * A hook that is called during app startup, from the `MainActivity`.
+ *
+ * Note that the hook is run in the background: it will not block startup of the app.
+ */
+fun interface AppStartupHook {
+    /** @param activity The activity which is active while the app starts (i.e. the `MainActivity`). */
+    suspend fun onAppStartup(activity: Activity)
+}