Added Auth0 device authorization flow example

[schoenenberg]

Hi,

thank you for this great library! I have used it to implement a CLI using Auth0. I would like to contribute this example back, because it took me some time to figure out a strange compiler error concerning the request_async method for the oauth2::DeviceAccessTokenRequest.

Best regards, Max

[Morishiri]

OMG. thank you very much for this!

Turns out the missing point to the one which already exists in the docs: https://docs.rs/oauth2/latest/oauth2/#device-code-flow

is

    .set_auth_type(AuthType::RequestBody);

[michaeljones]

Thank you for the this example. It is very helpful.