Pinned events : introduce banner formatter

libraries/eventformatter/api/src/main/kotlin/io/element/android/libraries/eventformatter/api/PinnedMessagesBannerFormatter.kt

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+/*
+ * Copyright (c) 2024 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
+ *
+ *     https://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.eventformatter.api
+
+import io.element.android.libraries.matrix.api.timeline.item.event.EventTimelineItem
+
+interface PinnedMessagesBannerFormatter {
+    fun format(event: EventTimelineItem): CharSequence
+}