refactor: event processing (#102)

Description

Refactoring Event processing:

• define the interface for Query Optimization assistant;
• define the interface for communication with an assistant;
• define the interface for getting user info;
• common components for an ability to extend communication channels: message processor, session and user managers;
• create a Slack communication channel;

Related issue

#102 (closed)

Checklist

• the MR description has been reviewed
• this MR contains text changes and they have been reviewed OR there are no texts changes
• this MR contains GUI/CLI changes and they have been reviewed OR there are no GUI/CLI changes
• this MR contains API changes and they have been reviewed OR there are no API changes

Closes #102 (closed)