Add the ability to set or redefine attributes for AMQPMessage.

[pmikle]

One of content_type, content_encoding, message_id, user_id, app_id, delivery_mode, priority, timestamp, expiration, type or reply_to, headers.

[pmikle]

Hello, @Seldaek !
Recently i have worked at #1724.
It works only when exchange instance of AMQPExchange.
But I need to add ability set custom attributes in another situations.
Would you also apply this pull request to main branch?
Please pay attention to this.
Thank you.

[Seldaek]

Thanks