More resilient droplet upload

[kathap]

Previously, the cc-uploader process did not have a draining mechanism. When the process was stopped, any ongoing droplet uploads were aborted immediately, leading to potential failures and incomplete uploads.

This PR introduces a graceful shutdown mechanism to allow ongoing uploads to complete before termination. The key enhancements include:

• Handling shutdown signals:
  • The process now listens for termination signals (SIGINT, SIGTERM) and responds by allowing uploads to finish before exiting.
• Tracking active uploads:
  • A global WaitGroup (uploadWaitGroup) is introduced to track ongoing uploads, incremented when a new droplet upload begins and decremented when it completes.
• Graceful waiting before shutdown:
  Upon receiving a shutdown signal, cc-uploader will:
  1 Log the shutdown event.
  2 Wait for all active uploads to complete using the uploadWaitGroup.
  3 Gracefully shut down the HTTP servers (TLS and non-TLS).
• Graceful server shutdown:
  Uses http.Server.Shutdown with a timeout to gracefully close both TLS and non-TLS servers.

Code Changes

1. Signal Handling:
   • Introduced a signalChan to capture termination signals.
2. WaitGroup for Uploads:
   • uploadWaitGroup ensures cc-uploader waits for all uploads to finish before stopping.
3. Graceful Shutdown Logic:
   • The shutdown process now waits for active uploads to complete and cleanly shuts down the HTTP servers.

• Links to any other associated PRs
  Add draining for cc_uploader capi-release#529
  Add draining for cc uploader cloud_controller_ng#4296