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**What this PR does / why we need it**:
sync code from copy destination
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
**Special notes for your reviewer**:
**Release note**:
```
NONE
```
This implements support for the new `auth-extra-groups` key in `bootstrap.kubernetes.io/token` secrets by adding extra groups to the user info returned for valid bootstrap tokens.
This adds constants and validation for a new `auth-extra-groups` key on `bootstrap.kubernetes.io/token` secrets. This key allows a bootstrap token to authenticate to extra groups in addition to the `system:bootstrappers` group.
Extra groups are always applied in addition to the `system:bootstrappers` group, must begin with a `system:bootstrappers:` prefix, are limited in length, and are limited to a restricted set of characters (alphanumeric, colons, and dashes without a trailing colon/dash).
