feat: add `PostFormMap` and `GetPostFormMap` to `RequestContext`

[snowykami]

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feat

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为RequestContext添加 PostFormMap 与 GetPostFormMap 方法

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en: Clients can use map for form submission in the request body's form-data or x-www-form-urlencoded, for example attr[k1]=v1&attr[k2]=v2&attr[k3]=v3. The method PostFormMap("attr") can be used to obtain the map {"k1": "v1", "k2": "v2", "k3": "v3"}. This method is implemented by referring to gin.Context.PostFormMap. Map can be used more conveniently for form data transmission.

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