Rework a bit MatrixClientHolder and reintroduce cacheIndex...

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/MatrixClientProvider.kt

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2023 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.element.android.libraries.matrix.api
+
+import io.element.android.libraries.matrix.api.core.SessionId
+
+interface MatrixClientProvider {
+    /**
+     * Can be used to get or restore a MatrixClient with the given [SessionId].
+     * If a [MatrixClient] is already in memory, it'll return it. Otherwise it'll try to restore one.
+     * Most of the time you want to use injected constructor instead of retrieving a MatrixClient with this provider.
+     */
+    suspend fun getOrRestore(sessionId: SessionId): Result<MatrixClient>
+}