* 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
* Add ksonnet version to version endpoint
I needed to move config.go out of the cli package to fix a circular dependency.
* Remove ksonnetVersion field from the ArgoCD version struct
* Support OAuth2 login flow from CLI (resolves#172)
* Refactor SessionManager to handle local and OAuth2 logins.
* argo login will request permanent credentials after OAuth2 flow
* Implement proper OIDC app state nonce. Add explicit `--sso` flag to `argo login`
* Don't ask for user credentials if username and password are specified as arguments
* Add cli image make target
* Don't re-prompt username/password in PromptCredentials
