Enhance the garbage collector to periodically refresh the resources it
monitors (via discovery) to enable custom resource definition GC.
This implementation caches Unstructured structs for any kinds not
covered by a shared informer. The existing meta-only codec only supports
compiled types; an improved codec which supports arbitrary types could
be introduced to optimize caching to store only metadata for all
non-informer types.
This adds an interface form of dynamic.Client and
dynamic.ResourceClient, making those two follow the general client
conventions: `Interface` is an interface, and `Client` is the concrete
implementation. `ClientPool` retains it's interface status.
This allows us to create a fake implemenation of dyanmic.Interface,
dynamic.ResourceInterface, and dynamic.ClientPool for testing.
Allow the list of resources the garbage collector controller should
ignore to be customizable, so downstream integrators can add their own
resources to the list, if necessary.
Automatic merge from submit-queue (batch tested with PRs 40055, 42085, 44509, 44568, 43956)
Fix gofmt errors
**What this PR does / why we need it**:
There were some gofmt errors on master. Ran the following to fix:
```
hack/verify-gofmt.sh | grep ^diff | awk '{ print $2 }' | xargs gofmt -w -s
```
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: none
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
