Target client improvements

[timebertt]

How to categorize this PR?

/area scalability robustness quality
/kind enhancement
/priority normal

What this PR does / why we need it:

This PR improves the following aspects of the client for the target cluster:

• The cache can be disabled via the newly added command line flag --target-disable-cache. In this case, grm won't cache any objects of the target cluster and always talk to the target API server directly (fixes Add option to disable target cache #92).
• Switches to a DynamicRESTMapper, which will ensure that resources are rediscovered on NoMatchErrors, but discovery calls are not executed more than once every 10 minutes (fixes Switch to DynamicRESTMapper for target client #93).

I took the chance to refactor the contents of app.go, as I found it to be very overloaded and hard to comprehend.
Now

• both the health and secret controller are placed in their own controller package
• controllers and the corresponding options are setup in the respective controller packages in add.go
• the health endpoint is moved into its own dedicated package
• the ClassFilter logic is moved into its own dedicated package
• all remaining general options (i.e. command line flags) are handled in dedicated files under cmd
• app.go only instruments those options and AddToManager functions to put everything together. This is very similar to how we construct manager, options and controllers in our extensions.

Also, I switched to our gcr copies of the base images.

Special notes for your reviewer:
Similar to g/g, grm now also vendors our c-r fork in order to get the fix from kubernetes-sigs/controller-runtime#1151:
sigs.k8s.io/controller-runtime => github.com/gardener/controller-runtime v0.6.3-gardener.1

We can drop it, once we have upgraded to [email protected].

Release note:

The cache of the kubernetes client for the target cluster can now be disabled via the `--target-disable-cache` flag.

gardener-resource-manager now uses a `DynamicRESTMapper`, which will reduce the amount of explicit discovery calls and faster reconciliation loops and some cases.

[timebertt]

Also tested this PR in my local gardener setup, shoot creation/reconciliation/hibernation/wakeup/deletion works fine.
Also did some load testing with large secrets like described in #92 and grm was completely stable and at a constant level of ~30MiB of Memory 💰

[timuthy]

/assign

[timuthy]

Nice PR. Now it looks very similar to what we do in the Gardener extension which will further simplify maintenance, thanks for that. I only found one nit for now.

[timuthy]

/lgtm