Add an easy way to retrieve a token given only the refresh token is valid

[Emilgardis]

should be easier than

match refresh_token
    .refresh_token(client, &client_id, client_secret.as_ref())
    .await
{
    Ok((a, _, r)) => UserToken::from_existing(client, a, r, client_secret)
        .await?,
    Err(e) => {
        panic!()
    }
}

perhaps a way to do UserToken::from_existing_or_refresh, or we could do that implicitly (I dislike that idea though)