Added Conversation Handler Feature

[gokhanaltun]

This feature facilitates step-by-step interaction with users during a conversation.

Here are the changes I made:

- conversation_handler.go: I created a new structure named ConversationHandler. This structure monitors the active stage of the conversation and calls functions according to specific stages.
- conversation_handler_test.go: I added a test file that tests the functionalities of the Conversation Handler.
- Added a usage example in the examples section.