Automatic merge from submit-queue (batch tested with PRs 56716, 52371). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Separate loop and plugin control

**What this PR does / why we need it**: Separate loop and plugin control in the kube-controller-manager.
Adding an "--external-plugin" flag to specify a plugin to load when
cloud-provider is set to "external". Flag has no effect currently
when the cloud-provider is not set to external. The expectation is
that the cloud provider and external plugin flags would go away once
all cloud providers are on stage 2 cloud-controller-manager solutions.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #52369 

**Special notes for your reviewer**:

**Release note**:
```release-note Added --external-plugin flag to kube-controller-manager to determine plugin when --cloud-provider is set to external.
```