I added a multipart feature

[JustFrederik]

Its basically just ureq_multipart, but without the converion to ureqs Request so it will work with wasm.

use ehttp::multipart::MultipartBuilder;
let builder = MultipartBuilder::new()
            .add_file("test","/home/feiy/Desktop/1.txt").unwrap()
            .add_text("name","value");
let request = ehttp::Request:multipart(url, builder);

[JustFrederik]

thank you! Can you please add some helpful docstrings to the key things? 🙏

Especially to Request::multipart and to MultipartBuilder

I added an example to request::multipart. The multipart builder already had some documentation, but is also used in the example

[JustFrederik]

Should I add the method setter?

[tmtbe]

This pr doesn't seem to have changed in a long time.