multipart for wasm32

[Gottox]

This PR introduces Request::bearer_auth for wasm32 and multipart data based on FormData

I marked this as WIP as it is currently at proof of concept stage.

solves #965.

[Gottox]

Most of the code is based on the async implementation.

I believe that async and wasm (maybe even blocking) can share more code in that regard.

[seanmonstar]

I think this looks great. What in particular do you think needs more work?

[seanmonstar]

This can probably be just a simple sentence, since the main docs use the non-wasm version.

[Gottox]

For me there's the issue that async_impl and wasm have lots of dublicated logic, only the serialisation to FormData for wasm and Request for async_impl are different.

Also, it is largely untested as the project I'd like to use this for is currently in the process of being ported to wasm.

Another issue is, that async_impl supports stacking multipart messages in eachother, my wasm implementation will fail during runtime when the user does this.

All in all only style questions and edge cases that I currently don't like. And as I said - basicly untested code.

[seanmonstar]

I hear you on the duplicated logic. It would be a good PR to try to consolidate as much as possible into some shared module. But we don't have to figure it out in this PR.

Also, if you know how we can setup unit testing for WASM, I'd be thrilled. I don't know too much about WASM testing, myself. See #657

[Gottox]

Well, actually I'm as new to wasm as you are, but I know that you can use wasm-pack to test the code.

[seanmonstar]

This seems good to me, if you want to learn how to test Rust WASM and deal with #657, that could be a cool next step. If not, no problem, thanks for this PR!

[Gottox]

Yay! Have fun!