* chore: Migrate CI to GitHub actions
* Do not install golangci-lint, we use the action
* Integrate codecov.io upload
* Use some better names for analyze job & steps
* go mod tidy
* Update tools
* Disable CircleCI completely
* Satisfy CircleCI with a dummy step until it's disabled
* Move utils packages that are required for gitops engine under engine/pkg/utils package.
Following changes were implemented:
* util/health package is split into two parts: resource health assessement & resource health assessement and moved into engine/pkg/utils
* utils packages moved: Closer and Close method of util package moved into engine/pkg/utils/io package
* packages diff, errors, exec, json, kube and tracing moved into engine/pkg/utils
* Move single cluster caching into engine/kube/cache package
* move sync functionality to engine/kube/sync package
* remove dependency on metrics package from engine/pkg/utils/kube/cache
* move annotation label definitions into engine/pkg/utils/kube/sync
* make sure engine/pkg has no dependencies on other argo-cd packages
* allow importing engine as a go module
* implement a high-level interface that might be consumed by flux
* fix deadlock caused by cluster cache event handler
* ClusterCache should return error if requested group kind not found
* remove obsolete tests
* apply reviewer notes
