Automatic merge from submit-queue (batch tested with PRs 49861, 50933, 51380, 50688, 51305)
Add configurable groups to bootstrap tokens.
**What this PR does / why we need it**:
This change adds support for authenticating bootstrap tokens into a configurable set of extra groups in addition to `system:bootstrappers`. Previously, bootstrap tokens could only ever authenticate to the `system:bootstrappers` group.
Groups are specified as a comma-separated list in the `auth-extra-groups` key of the `bootstrap.kubernetes.io/token` Secret, and must begin with the prefix `system:bootstrapper:` (and match a validation regex that checks against our normal convention). Whether or not any extra groups are configured, `system:bootstrappers` will still be added.
This also adds a `--groups` flag for `kubeadm token create`, which sets the `auth-extra-groups` key on the resulting Secret. The default is to not set the key.
`kubeadm token list` is also updated to include a `EXTRA GROUPS` output column.
**Which issue this PR fixes**: fixes#49306
**Special notes for your reviewer**:
The use case for this is in https://github.com/kubernetes/kubernetes/issues/49306. Comments on the feature itself are probably better over there. It will be part of how HA/self-hosting kubeadm bootstraps new master nodes (post 1.8).
**Release note**:
```release-note
Add support for configurable groups for bootstrap token authentication.
```
cc @luxas @kubernetes/sig-cluster-lifecycle-api-reviews @kubernetes/sig-auth-api-reviews
/kind feature
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