api interface: add a trigger func

[platx]

It could be useful in some scenarios when you want to trigger another handler from some specific handler, which doesn't have a link to a Bot struct, but has just a context provided.

For example:

func SomeHandlerFunc(c telebot.Context) error {
    // some business logic
    
   return c.Bot().Trigger("/another-command", c)
}

The original method was implemented here: #687