Add support for handling additional properties in Form request

[hamzamahmood]

Describe the bug
Currently, the additional properties in a model annotated with [JsonExtensionData] are not being flattened with the model's members when sent through the form.

Expected behavior
The properties set through the additional properties dictionary with [JsonExtensionData] should be flattened with the model's members when sent through the form.

To Reproduce
Steps to reproduce the bug:

1. Create a model containing a Dictionary<string, object> property.
2. Annotate the dictionary property with [JsonExtensionData].
3. Make an API call with both form data and a body that includes an instance of this model.
4. Send the request to the API server. Notice that, while the body contains the additional properties in a flattened structure, the form data sends the additional properties as a dictionary instead of flattening them.