Feature: add a callback example

[luhaoling]

Checklist

• I've searched for similar issues and couldn't find anything matching
• I've discussed this feature request in the OpenIMSDK Slack and got positive feedback

Is this feature request related to a problem?

None

Problem Description

Solution Description

Add a callback example to let people know how to use callback. When you send a text message or picture to the rob account, the rob will return this text message or picture to you.

Benefits

Good for letting people know how to use callback

Potential Drawbacks

No response

Additional Information

No response