[swift5] Add UUID support in FormDataEncoding

Created by: kalinjul

For an API POST request with content type "multipart/form-data" and an parameter of format": "uuid",, openapi-generator correctly generates an API function with UUID as parameter, but crashes when calling it, because FormDataEncoding does not support UUID types.

This PR adds support for UUID in FormDataEncoding for Alamofire and URLSession.

I have no idea if, how or where to add tests for this.

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@4brunu: mentioning you for review