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 Migrate FeatureGates type of kube-proxy from string to map[string]bool

**What this PR does / why we need it**:
Migration of FeatureGates type. This is a follow-up of #53025.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
ref: #53025
https://github.com/kubernetes/kubernetes/pull/57754#discussion_r160023416

**Special notes for your reviewer**:
/cc @luxas @mtaufen @ncdc 

**Release note**:

```release-note
action required: kube-proxy: feature gates are now specified as a map when provided via a JSON or YAML KubeProxyConfiguration, rather than as a string of key-value pairs.
```