Refactor Agent to use DialogueKit

What's changed?

The class Agent was renamed to MovieBotAgent and inherits from dialoguekit.participant.Agent.
The class MovieBotDialogueConnector inherits from dialoguekit.connector.DialogueConnector and handles the connection between agents and users.
The class DialogueContext was removed. The previous utterances can be accessed directly in the dialogue manager. The list of movies recommended is now part of DialogueState.
The class Controller inherits from dialoguekit.platforms.Platform.

Follow up includes the creation of tests for the class MovieBotAgent.

Depends on #167