Merge pull request #46254 from mtaufen/dkcfg

Automatic merge from submit-queue (batch tested with PRs 50016, 49583, 49930, 46254, 50337)

Alpha Dynamic Kubelet Configuration

Feature: https://github.com/kubernetes/features/issues/281

This proposal contains the alpha implementation of the Dynamic Kubelet Configuration feature proposed in ~#29459~ [community/contributors/design-proposals/dynamic-kubelet-configuration.md](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/dynamic-kubelet-configuration.md). 

Please note:
- ~The proposal doc is not yet up to date with this implementation, there are some subtle differences and some more significant ones. I will update the proposal doc to match by tomorrow afternoon.~
- ~This obviously needs more tests. I plan to write several O(soon). Since it's alpha and feature-gated, I'm decoupling this review from the review of the tests.~ I've beefed up the unit tests, though there is still plenty of testing to be done.
- ~I'm temporarily holding off on updating the generated docs, api specs, etc, for the sake of my reviewers 😄~ these files now live in a separate commit; the first commit is the one to review.

/cc @dchen1107 @vishh @bgrant0607 @thockin @derekwaynecarr 

```release-note
Adds (alpha feature) the ability to dynamically configure Kubelets by enabling the DynamicKubeletConfig feature gate, posting a ConfigMap to the API server, and setting the spec.configSource field on Node objects. See the proposal at https://github.com/kubernetes/community/blob/master/contributors/design-proposals/dynamic-kubelet-configuration.md for details.
```

hack/verify-flags/known-flags.txt

@@ -194,6 +194,7 @@ drop-embedded-fields
 dry-run
 dump-logs-on-failure
 duration-sec
+dynamic-config-dir
 e2e-output-dir
 e2e-verify-service-account
 enable-aggregator-routing
@@ -349,6 +350,7 @@ image-service-endpoint
 included-types-overrides
 include-extended-apis
 include-extended-apis
+init-config-dir
 initial-sync-timeout
 input-base
 input-dirs
@@ -493,6 +495,7 @@ network-plugin
 network-plugin-dir
 network-plugin-mtu
 node-cidr-mask-size
+node-config-dir
 node-eviction-rate
 node-instance-group
 node-ip