Recently, telegram has published [Telegram Bot API Server] - HTTP API <->
MTPROTO proxy which is used internally by telegram. Now it's possible to run
the server locally.
However currently it's not possible to use teloxide with such local server,
because API url is hardcoded into teloxide.
This commit makes _internall_ changes to allow setting the API url. It doesn't
yet allow to set the url from user code (i.e.: no changes in public API), in my
opinion such additions are blocked on some bot refactorings.
`Bot::api_url` is made `Option<Arc<str>>` to minimize costs in case of 'default
url' (probably the most common case).
Progress of this issue is tracked in https://github.com/teloxide/teloxide/issues/317
[Telegram Bot API Server]: https://github.com/tdlib/telegram-bot-api
