[Add] Implement Slack Socket Mode support

[JCMarques15]

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Description

This PR introduces support for Slack Socket Mode, enabling real-time message processing using websockets. This feature enhances the Slack integration by allowing for more efficient and responsive communication, and a way to bypass ingress restrictive networks.

Changes

• Added Slack Socket Mode handler in server.py
• Created start_socket_mode function in slack/app.py
• Implemented conditional logic to use Socket Mode when SLACK_WEBSOCKET_TOKEN is set
• Added an example script slack_websocket_test.py for testing the Slack websocket handler

How to Test

1. Set the following environment variables:
   • SLACK_BOT_TOKEN
   • SLACK_SIGNING_SECRET
   • SLACK_WEBSOCKET_TOKEN
2. Run the application and verify that the Slack integration connects using Socket Mode
3. Test sending messages through Slack and ensure they are processed correctly

Additional Notes

• The existing HTTP handler remains active when only SLACK_BOT_TOKEN and SLACK_SIGNING_SECRET are provided
• When SLACK_WEBSOCKET_TOKEN is set, the application will prioritize Socket Mode over the HTTP handler