Add basic pinned message support.

app/src/main/java/org/thoughtcrime/securesms/mms/OutgoingMessage.kt

@@ -499,6 +499,17 @@ data class OutgoingMessage(
       )
     }
 
+    @JvmStatic
+    fun pinMessage(threadRecipient: Recipient, sentTimeMillis: Long, expiresIn: Long, messageExtras: MessageExtras): OutgoingMessage {
+      return OutgoingMessage(
+        threadRecipient = threadRecipient,
+        sentTimeMillis = sentTimeMillis,
+        expiresIn = expiresIn,
+        messageExtras = messageExtras,
+        isSecure = true
+      )
+    }
+
     @JvmStatic
     fun quickReply(
       threadRecipient: Recipient,