Add support for Telegram sponsored messages

[d99kris]

From https://core.telegram.org/api/sponsored-messages#sponsored-messages-in-third-party-apps

Telegram continues to grow worldwide, in part thanks to third-party apps using the Telegram API. To cover the increasing costs that come with this growth, Telegram added sponsored messages – a paid privacy-friendly way to promote bots and channels.

If their app allows its users to access content from Telegram channels, third-party developers using the Telegam API are required to support and properly display official sponsored messages in their apps by January 1, 2022.

This issue is for adding support for sponsored messages in nchat.

For testing there is a channel with sponsored messages at https://t.me/SecretAdTestChannel

References: https://github.com/tdlib/td/issues/1753 https://github.com/tdlib/td/issues/1759

[d99kris]

Conceptual design:

• Tgchat to determine chat type when calling getChat and store in a local map.
• Add new action SetCurrentChatRequest for ui to notify tgchat when current chat changes.
• When SetCurrentChatRequest is called on a chatTypeSupergroup which is_channel_, tgchat shall call getChatSponsoredMessages (ensure 5 mins interval between calls).
• Tgchat can pass sponsored messages as regular chat messages to ui, with some special properties, like timestamp max_int (to be shown last), flag to prevent caching and a link.
• When sending read receipt for such a message, tgchat will have special handling to call viewSponsoredMessage instead of regular markMessageReadRequest.