Creates a startDM method so we can reuse it for the new flow

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

@@ -0,0 +1,43 @@
+/*
+ * 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.room
+
+import io.element.android.libraries.matrix.api.MatrixClient
+import io.element.android.libraries.matrix.api.core.RoomId
+import io.element.android.libraries.matrix.api.user.MatrixUser
+
+/**
+ * Try to find an existing DM with the given user, or create one if none exists.
+ */
+suspend fun MatrixClient.startDM(matrixUser: MatrixUser): StartDMResult {
+    val existingRoomId = findDM(matrixUser.userId)?.use { existingDM ->
+        existingDM.roomId
+    }
+    return if (existingRoomId != null) {
+        StartDMResult.Success(existingRoomId, isNew = false)
+    } else {
+        createDM(matrixUser.userId).fold(
+            { StartDMResult.Success(it, isNew = true) },
+            { StartDMResult.Failure(it.localizedMessage) }
+        )
+    }
+}
+
+sealed interface StartDMResult {
+    data class Success(val roomId: RoomId, val isNew: Boolean) : StartDMResult
+    data class Failure(override val message: String?) : StartDMResult, Exception(message)
+}