[FEATURE REQUEST] RestfulAPI supports extended fields when sending messages

[onlyGuo]

Why this feature?
The extension field is used quite frequently and is very important. For example, it can store VPI information, user levels, chat bubble skins, and other additional attributes. Without it, personalized development would be difficult.

Suggested Solution
Add support for passing extension fields in the RestFul interface and update the documentation.

Additional Information
The extension field referred to here is the 'ex' field found at https://docs.openim.io/zh-Hans/restapi/webhooks/msg/sendSingleMsgAfter

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