Automatic merge from submit-queue. 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>.

Make feature gates loadable from a map[string]bool

Command line flag API remains the same. This allows ComponentConfig             
structures (e.g. KubeletConfiguration) to express the map structure             
behind feature gates in a natural way when written as JSON or YAML.             
                                                                                
For example:                                                                    
                                                                                
KubeletConfiguration Before:
```
apiVersion: kubeletconfig/v1alpha1
kind: KubeletConfiguration
featureGates: "DynamicKubeletConfig=true,Accelerators=true"
```

KubeletConfiguration After:
```
apiVersion: kubeletconfig/v1alpha1
kind: KubeletConfiguration
featureGates:
  DynamicKubeletConfig: true
  Accelerators: true
```

Fixes: #53024

```release-note
The Kubelet's feature gates are now specified as a map when provided via a JSON or YAML KubeletConfiguration, rather than as a string of key-value pairs.
```

/cc @mikedanese @jlowdermilk @smarterclayton