Idea for a new way to handle conversations

[Kotov584]

Is your feature request related to a problem? Please describe.
I'm always frustrated when I have to define conversations using input handlers.
When you are using input handlers, it's pretty hard to write and manage complex conversations.

Describe the solution you'd like
Instead of input handlers, we could use classes for defining conversations. So we are able to write more well-architected code, even when it is complex.

Example:

package com.example.blank.conversation

import eu.vendeli.tgbot.TelegramBot
import eu.vendeli.tgbot.api.message
import eu.vendeli.tgbot.types.User
import eu.vendeli.tgbot.types.internal.ProcessedUpdate

sealed class GreetingsConversation {
    object Start : GreetingsConversation() {
        fun handle(update: ProcessedUpdate, user: User, bot: TelegramBot) {
            message("Hello, let's move to second step!").send(user, bot)
            //set secondStep state
           //maybe something like 
           setData(user, "hello" to update.text)
           setState(user, GreetingsConversation::start)
        }
    }

    object SecondStep : GreetingsConversation() {
        fun handle(update: ProcessedUpdate, user: User, bot: TelegramBot) {
            // retrieve data from start step and proceed
            message("Nice job! You completed this.").send(user, bot)
            deleteState(user, GreetingsConversation)
        }
    }
}

Additional context
Not sure, if we can do this way.
We discussed about it in Telegram group.

[Kotov584]

or also this

package com.example.blank.conversation

import eu.vendeli.tgbot.TelegramBot
import eu.vendeli.tgbot.api.message
import eu.vendeli.tgbot.types.User

class GreetingsConversation {
    fun start(user: User, bot: TelegramBot) {
        message("Hello, let's move to second step!").send(user, bot)
    }

    fun secondStep(user: User, bot: TelegramBot) {
        message("Nice job! You completed this.").send(user, bot)
    }
}

[vendelieu]

Experimental implementation is released in #61

[vendelieu]

implemented in #75