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unpack dynamic kubelet config payloads to files

This PR unpacks the downloaded ConfigMap to a set of files on the node.

This enables other config files to ride alongside the
KubeletConfiguration, and the KubeletConfiguration to refer to these
cohabitants with relative paths.

This PR also stops storing dynamic config metadata (e.g. current,
last-known-good config records) in the same directory as config
checkpoints. Instead, it splits the storage into `meta` and
`checkpoints` dirs.

The current store dir structure is as follows:
```
- dir named by --dynamic-config-dir (root for managing dynamic config)
| - meta (dir for metadata, e.g. which config source is currently assigned, last-known-good)
  | - current (a serialized v1 NodeConfigSource object, indicating the assigned config)
  | - last-known-good (a serialized v1 NodeConfigSource object, indicating the last-known-good config)
| - checkpoints (dir for config checkpoints)
  | - uid1 (dir for unpacked config, identified by uid1)
    | - file1
    | - file2
    | - ...
  | - uid2
  | - ...
```

There are some likely changes to the above structure before dynamic config goes beta, such as renaming "current" to "assigned" for clarity, and extending the checkpoint identifier to include a resource version, as part of resolving #61643.

```release-note
NONE
```

/cc @luxas @smarterclayton