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Expose single annotation/label via downward API

**What this PR does / why we need it**:

https://github.com/kubernetes/community/blob/master/contributors/design-proposals/node/annotations-downward-api.md

Support exposing single annotation via both env and volume downward API using the following syntax:

```
metadata.annotations['key']
metadata.labels['key']
```

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

#31218

**Special notes for your reviewer**:

This PR takes over the work in https://github.com/kubernetes/kubernetes/pull/41648.

**Release note**:

```
A single value in metadata.annotations/metadata.labels can be passed into the containers via Downward API
```

/assign @thockin @vishh