Add underpinnings to allow for local plaintext export.

Co-authored-by: Cody Henthorne <cody@signal.org>

lib/archive/src/main/java/org/signal/archive/stream/JsonBackupWriter.kt

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.archive.stream
+
+import org.signal.archive.proto.BackupInfo
+import org.signal.archive.proto.Frame
+import org.signal.core.util.toJson
+import java.io.IOException
+import java.io.OutputStream
+
+/**
+ * Writes backup frames to the wrapped stream as newline-delimited JSON (JSONL).
+ */
+class JsonBackupWriter(private val outputStream: OutputStream) : BackupExportWriter {
+
+  @Throws(IOException::class)
+  override fun write(header: BackupInfo) {
+    outputStream.write((header.toJson() + "\n").toByteArray())
+  }
+
+  @Throws(IOException::class)
+  override fun write(frame: Frame) {
+    outputStream.write((frame.toJson() + "\n").toByteArray())
+  }
+
+  override fun close() {
+    outputStream.close()
+  }
+}