fix: add prerequisite check and auto-install for essential tools

Issue:
- Script fails with 'curl: command not found'
- Assumes curl, wget, gpg are pre-installed
- No graceful handling of missing prerequisites

Fix:
- Check for essential tools at script start: curl, wget, gpg
- Auto-install missing tools via apt-get
- Handle both root and non-root execution
- Clear error messages if installation fails

Prerequisites checked:
✓ curl - Used for downloading GPG keys, installers
✓ wget - Used for downloading Go, protoc, etc.
✓ gpg - Used for dearmoring GPG keys
✓ ca-certificates - Required for HTTPS downloads

Behavior:
1. Script detects missing tools
2. Shows warning: 'Missing essential tools: curl wget gpg'
3. Attempts automatic installation via apt-get
4. If successful: continues normally
5. If failed: exits with clear error message

Benefits:
- ✅ Works in minimal base images
- ✅ Self-healing for common missing dependencies
- ✅ Clear error messages for manual intervention
- ✅ No assumptions about pre-installed tools

Note: Your Dockerfile includes these tools, but this adds
resilience for testing in minimal environments.