Feat/async trait auth

[dizda]

Based on the work of #36 (thank you)

More flexibility has been added, such as

• async_trait is being used, which makes it easier to use async fn for authentication
• allow_no_auth flag, for scenario where we could auth the client via other parameters such as IP address, etc. so the client doesn't need to send credentials, but will be checked by the Authentication trait anyway.
• user's account can be returned once the socket has been successfully upgraded to socks5 (hence successfully authenticated), so no need to use black magic anymore to do so

[dizda]

Feel free to review @nemosupremo

[nemosupremo]

Ah looks good, I was a bit confused on how you planned to return the users credentials and spent some time on that, but the get_credentials makes more sense.

Only thing I would add is maybe SimpleUserPassword should return an AuthenticationMethod::Password like struct so that users who might depend on AuthenticationMethod::Password with the username and password have a clear upgrade path.

[dizda]

Only thing I would add is maybe SimpleUserPassword should return an AuthenticationMethod::Password like struct so that users who might depend on AuthenticationMethod::Password with the username and password have a clear upgrade path.

Thanks for the feedback, I've updated the server example accordingly.

[dizda]

Released on 0.9.0