Kubernetes backend should use informer cache for retrieving resources

[jannfis]

Is your task related to a problem? Please describe.

Right now, the Kubernetes backend for Applications and AppProjects talk directly to the Kubernetes API server when retrieving (e.g. list or get) resources. However, since we use shared informers for both type of resources, we should make use of its caches instead. This will reduce the number of requests being made to the Kubernetes API significantly.

Describe the solution you'd like

The Get and List methods of the Kubernetes backends for Application and AppProject type resources should use the informer's cache instead of reaching out to the API.

Describe alternatives you've considered

Additional context