Use true JSON library to configure Telegram

[denis-stepanov]

Currently Telegram configuration, such as keyboards, represent JSON manually crafted in the code. For complex arrays it quickly becomes cumbersome to manage, so the task would be to switch to use of a real JSON library. Because the underlying Telegram bot library already includes JSON library as a dependency, the change should not result in more dependencies or make the executable size grow significantly.

NB: Google Assistant support includes a tiny portion of JSON to broadcast a message, also manually crafted. This case could be left as is in order to avoid extra dependencies to build 2-3 lines of JSON.