Add OnPreShutdown Extension point

[jeevatkm]

The goal is to add new OnPreShutdown extension point in the HTTP engine.

Current Flow:

• Receive SIGINT/SIGTERM
• Trigger graceful shutdown with configured timeout on aah server
• Publish OnShutdown
• Clean exit

After this enhancement, the flow would be:

• Receive SIGINT/SIGTERM
• Publish OnPreShutdown
• Trigger graceful shutdown with configured timeout on aah server
• Publish OnShutdown
• Clean exit

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• [x] Implementation
• [x] Documentation

[jeevatkm]

It done 😄