Implement DCF (Device Code auth flow)

[Emilgardis]

We should implement this with the correct grants.

I'm a bit conflicted though about how the interface should look like. Maybe a builder like the other auth flows, but then two different methods for public or confidential type.

We also need to implement refresh tokens without client_secret. We'd have to either change

twitch_oauth2/src/lib.rs

Line 244 in bf0eada

client_secret: &ClientSecret,

or provide two new functions

[bryanveloso]

I'm opting to use DCF in my desktop app and have had to use the popular oauth2 crate for it. Unfortunately, Twitch (surprise) doesn't follow the spec, which requires the 400 statuses to return the auth status in an error property. Twitch in this case returns the status in a message property which panics the crate. So I'm having to manually do a lot of this.

I'd love to have to not deal with it, but unfortunately I'm too new of a Rust dev to really contribute otherwise. I do like how painless oauth2's method of implementing DCF is though!

[Emilgardis]

I think something like the following will work

let mut builder = DeviceUserTokenBuilder::new(client_id, scopes);
if confidential {
    builder.set_secret(client_secret);
}
let code = builder.exchange_device_code(client).await?;
println!("Enter {code}");
...
let token: UserToken = UserTokenbuilder.finish().await?;