feat: support option to mark messages unread by message timestamp (#1648)

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## Description of the changes, What, Why and How?
When marking messages as unread, it is now possible to pass a timestamp
to mark all messages after this time as unread for a channel.

## Changelog

-

Author: ianb-gs

src/types.ts

@@ -1262,6 +1262,7 @@ export type MarkUnreadOptions = {
   connection_id?: string;
   message_id?: string;
   thread_id?: string;
+  message_timestamp?: string | Date;
   user?: UserResponse;
   user_id?: string;
 };