Notifier[telegram]: Add proxy support for Telegram notifications

[hosni]

Hi Thanks for maintaining this great project!

I'd like to request a new feature for the Telegram notifier: support for using a proxy (such as SOCKS5 or HTTP). This is particularly important for users in countries like Iran and China, where Telegram is blocked and cannot be accessed directly without a proxy.

It would be great if the Telegram notifier could optionally accept a proxy parameter so users can configure it like this:

notifiers:
  telegram:
    type: telegram
    chat_id: '@gobackuptest'
    token: this-is-your-bot-token
    proxy: socks5://127.0.0.1:1080

This would make the notifier usable in more regions and environments.