Refactor to Go Workspaces and add more tests

[karlkfi]

This PR transitions the project to a Go workspace architecture using go.work, enabling better management of the multiple modules in this repository (kubexit and test-server). It also introduces a structured CI/CD pipeline via GitHub Actions, providing dedicated workflows for unit, integration, and end-to-end testing.

Key Changes

Architecture & Dependency Management

• Go Workspace Transition: Implemented go.work to manage the root module, cmd/kubexit, and cmd/test-server as part of a single workspace. This simplifies development across multiple modules.
• Dependency Updates: Upgraded various Go dependencies (e.g., pflag) and added essential utilities like errgroup and testify.
• Module Cleanup: Introduced scripts/go-modules.sh to streamline the process of updating and removing unused Go modules across the workspace.

CI/CD Pipeline Enhancements

• GitHub Actions Workflows: Established a robust testing pipeline with separate workflows for:
  • Unit Testing: (.github/workflows/unit-test.yaml) runs the project's unit test suite.
  • Integration Testing: (.github/workflows/integration-test.yaml) validates core functionality against Kubernetes-like environments.
  • End-to-End (E2E) Testing: (.github/workflows/e2e-test.yaml) provides full lifecycle validation of the kubexit process.
• Improved Documentation: Updated .github/workflows/README.md and ci/e2e-test/README.md to guide developers through the new CI processes.

Core Logic & Testing Improvements

• Refactored Kubernetes Watching: Improved the robustness of the watch mechanism in pkg/watch/watch.go by implementing a more reliable precondition check and refined event handling for pod lifecycle changes.
• Expanded Test Coverage:
  • Added comprehensive new unit tests for the supervisor (pkg/supervisor/supervisor_test.go) and tombstone (pkg/tombstone/tombstone_test.go) packages.
  • Enhanced the integration test suite with new test cases in cmd/test-server.

Test Plan

• Run unit tests via make test to ensure core logic remains intact.
• Verify integration tests pass using the newly established integration workflow.
• Confirm that the Go workspace structure correctly builds both kubexit and test-server binaries.