Cleanup context handling

[ensonic]

1. Modernized Graceful Shutdown: Updated all entry-point main.go files to use os/signal.NotifyContext. This provides a base context that is automatically cancelled upon receiving termination signals (SIGINT/SIGTERM), allowing for graceful cleanup of all downstream components.
   • Updated: setup-dev, setup-robot, token-vendor, cr-syncer, http-relay-client, metadata-server, and gcr-credential-refresher.
2. Context Propagation in Controllers: Refactored the Kubernetes controllers (chart-assignment-controller and app-rollout-controller) to use signals.SetupSignalHandler() for their base context, which is then passed down to reconcilers and background release managers.
3. Refactored Library Functions: Updated configutil.ReadConfig and Synk.Init to accept a context.Context as their first parameter. This ensures that these long-running or network-dependent operations can be cancelled by their callers.
4. Eliminated Deprecated Contexts: Replaced the obsolete golang.org/x/net/context with the standard library's context package in cr-syncer.
5. Audit of context.TODO() and context.Background():
   • Confirmed that context.TODO() is not used (only in generated code).
   • Reduced the usage of context.Background() in production code, replacing it with passed-down contexts to ensure proper cancellation propagation.
6. Cleanup of tests to use t.Context()

[ensonic]

Sorry for the large PR, the majority is using t.Context() in tests though.