Provide method to get Vec<types::BotCommand> for set_my_commands()

[robinhundt]

The Telegram API offers the set_my_commands method to set the current commands automatically instead of manually through the BotFather.
This API endpoint is already implemented in the crate (docs) but it accepts a Vec<types::BotCommand>. Currently a user of the library would need to construct these BotCommands themselves. I propose to add a method to the utils::command::BotCommand trait (maybe commands(&self) -> Vec<BotCommand>) which returns a Vec of these BotCommands that can be sent to the API.
This would allow a user of the library to have the BotCommand enum declaration as the single source of truth and automatically send the correct command information to the API via set_my_command.

[robinhundt]

I suppose the implementation of that method would be similar to the implementation of the description method here:
https://github.com/teloxide/teloxide-macros/blob/30024d7792b27e1bf9fcdb61c6fe8555d6f0ee2a/src/lib.rs#L260-L296

[hirrolot]

Closed by #498.